This is a fairly interesting question that gets asked relatively often, so I decided to write a bit of an in-depth explanation as to my view on the subject.

First of all, I should explain that I'm in a bit of a weird spot when it comes to answering such questions, because I mostly test the mod with a custom tuning pack and some extra scripted features too. The tuning pack is quite vanilla-like, so I can update the mod without any issue while doing most testing with somewhat different conditions, but more specific questions like this are a bit tough to answer with great accuracy.

So, to start off, I do think the mod is decent practice, and substantially better than playing against vanilla AI. However, there are a plethora of caveats and things to consider when it comes to this.

Most importantly, it depends highly on your skill level. If you need to choose a very high difficulty, the resource bonuses are quite large and the gameplay becomes completely different. I play a different version now like I said, but I used to be able to beat it with even higher bonuses than it has now, and I had a top player report the same. Problem is, it's a grind, it's kind of pointless to call that a real COH3 match, it's an attritional by-numbers unit efficiency check. At best it's possible to cheese the AI out with a quick VP rush, then it's a bit more like a real game, but still not ideal.

However, I'm talking about a high level of gameplay here, like top 100 and above on 1v1 ladder.

In the custom tuning pack that we play, I can set the AI bonuses way lower and still get challenged (about half as high as base mod). I have a few people who play 1vAI quite often - the results with lower bonuses are much, much better. It's actually not that different practice-wise from playing against a real player in that case, in my opinion. At least as long as you're not specifically honing strategies that will only be effective against an AI opponent.

Sure, AI's unit utilization and movement is still very far from what a top player would do, but overall I'd say it's reasonably competent and the games are quite fun. So the main issue is the bonuses. If you can select bonuses which aren't too high, but still provide a good challenge where you win 50-80% of the time, but not always, it will give you pretty decent practice. If the bonuses are low enough then it will feel more or less like a normal game, even if with way fewer surprises and daring plays from the AI opponent.

On some occasions I had people report that they can't defeat even Easy difficulty - that difficulty even receives penalties, so obviously one should be able to beat it to have any hope of defeating stronger human opponents. Of course on the flipside is the fact that matchmaking will find you equal opponents regardless of your skill level, so you can avoid the frustrations of fighting a robotic opponent and just enjoy the game, and honestly it would always be my first recommendation for improvement - just queue and play.

Despite doing things somewhat differently from a human, it still does most things competently enough, even if you try a very meta strategy against it, AI will adapt to a decent degree. And just in general, mod's AI at least utilizes many different game concepts like unit preservation, rushing out tech, building a balanced unit composition, properly retreating, countering enemy units etc. It'll try to cut off your territory, it can rush out vehicles sometimes, it also has special logic for how it fights team weapons, how it uses vehicles, its mine usage is kind of scary good I think. I saw people point out recently that game AIs will always walk into defensive positions over and over - actually this is much improved in the mod - AI will avoid defensive positions + bombard the crap out of them, it'll wait for more friendly units to arrive before trying for a breakthrough in a fortified area. The mod does a lot, even if it's impossible to fully replicate human behavior.

AI is very good at dodging abilities and generally being aware of its units, which will also potentially put your multitasking to the max. The recently added AI action speed modifier slider, which lets you somewhat limit AI's APM basically, could also in theory let you overwhelm AI with your own multitasking, reduce its speed so this AI 'artificial' strength isn't quite as pronounced anymore. Because of course this can also be a point of frustration for newer players, the excessive multitasking can feel overwhelming. Although unfortunately there's not much I can do to limit this organically without implementing entirely new systems specifically for it, beyond the action speed slider.

As far as AI's weaknesses - even if AI does rush out a vehicle, it won't be nearly as efficient with it as a human would be. Even if it cuts off your territory, it won't capitalize on it if the opportunity presents itself to defend that cutoff nearly as well as a human would. It basically has no idea how to abuse buildings on the map. It moves around the map in a somewhat logical manner, but it won't be nearly as efficient in that regard as a human. It can't really properly switch between defensive and offensive playstyles. It has very little predictive capability, so it can't setup AT gun ambushes or ambushes in general.

Generally speaking, it'll execute just a very solid safe standard playstyle and apply pressure around the map rather evenly, it has a very limited idea of what map features are actually there, how to maximize army synergies "multiplicatively", how to really abuse light vehicle mobility etc. Somebody pointed out that it will "play the same" every time, and yea that's kind of true - there's lots of randomness to it, but it's kind of dumb randomness while still adhering to the same general rules all the time. The new personality settings help with that a tiny bit, but not much.

Still, I think self-improvement-wise it's a fairly good idea to first learn to be efficient and learn to play a safe playstyle rather than focusing on surprising your opponent, especially in a game like COH. Sure, for more veteran players it can get tedious quickly. I personally extract my fun by constantly switching my strategies against the AI, not so much by adapting to what the AI is doing. I think that's decent enough practice, the AI will react to you and one can do the initial refinement of that strategy's strengths, timings and fix pitfalls, in that sense preparing oneself for facing a real opponent who will try to exploit the same holes or be forced to defend against the strategy's strengths.

So the main point is that yes, the AI is competent enough to provide decent pvp practice, even if it's far from ideal. The only requirement is that you are being challenged by the mod without needing to select a very high difficulty (Hard, but not Expert). Try to focus on your mechanics while executing strategies that you would be executing in a pvp game, as you see it. Don't try to min-max your vs-AI strategy and utilize that for pvp, it's generally not going to translate 1:1.

I think overall if you're trying to improve, the main thing is to be mindful of what you're doing in your games - a sheer volume of games, pvp or otherwise, won't help you improve quickly. Try and optimize your timings, your unit movement, and steal strategies from the best players. I really recommend watching top players play, analysing your own replays, watching tightrope videos, maybe watching some tutorials that relic released on their channel if you're a complete beginner, so that you know what you're practicing for. Sure, best way is to play proper pvp, don't give in to ladder anxiety if that's the only reason you want to play vs AI, but there are plenty of reasons to want a more relaxed skirmish experience, especially for beginners, and I think the mod provides a decent enough experience for this in the end, and of course I'll keep improving it.

I basically haven't played proper COH pvp for years before COH3 release, mostly playing against this AI, and was able to reach tops of COH3 ladder with relative ease, heh. I do have opportunities to practice against some really strong opponents from time to time and a ton of experience in RTS genre in general, so no I am not claiming that this AI will get you to that point all on its own, but I do believe it's quite good for keeping the rust off.