Harkonnen suck

[mazrados]

.. Or I don't understand how to play this faction.

Opression is kind of useful but it's too much micromanagement. Combat drugs is useless for Cerberus because it makes them to "clone" and die instantly. And on top of that, they even did not scratch Fremen base.

[View The Phenom]

Oppression boosts the productivity of your villages immediately at the cost of reduced performance and a chance for Rebellion for a short period afterwards. It's really good for getting villages online quickly, or gaining access to large amounts of X resource when you really need it NOW.
There are also councilors who benefit from those rebellions initiating (Feyd-Rautha), and it's free experience for your military units. Just park some units on an oppressed village and forget about it.

Combat Drugs boost your units above the opposing army at the minor cost of -10% HP for the day. Generally, battles aren't going to last for longer than a day so the extended drain during combat is irrelevant. If you want to overwhelm an otherwise equal opposing army, Combat Drugs will do it.
And Iakid giving you cheaper Drugs means you're able to access it more often than anyone else. Not to mention that losing X unit isn't a complete loss since he gives you 60% of the cost back as well.

Harkonnen are probably the strongest faction in the game, so you're likely not exploiting their benefits properly. Bases are meant to be tanky, the inability of X to melt it immediately doesn't make that faction bad.

[mazrados]

Health was drained in a moment with combat drugs. Much faster then a day.

When I played Fremen, I destroyed Smugglers base with no problem. Yes, it's tanky and it takes time, but you can see damage over time and eventually destroyed base. In this case there was no damage at all. Literally, this Harkonnen units did not make any damage to undefended base.

[Katitoff]

You'll never deal any good damage to HQs if you won't bring gunners to strip armor first.
Its 20-40 armor depending on upgrades.