Played this game a ton on mobile.

TLDR: If you are a 40k fan, not hyper competitive, want something other than HS that is more board-centric and less cartoony in theme - Legions is for you.

- Lots of Warhammer themed characters.
- Lots of different play styles that are actually themed around and correlate very well with 40k factions (I sometimes find myself role-playing when building themed decks).
- Different warlords (there are ~200) bring so much variety compared to HS's 11 classes and their sub-classes.
Warlords are an actual units on board with attack stats, traits and spells exclusive to them. There are a lot of possibilities in deck building and a good chance that your design will go far in the ladder.
- Interesting Event (Arena) mode with selection of warlords that are themed around specific 40k scenarios.
- Great (at times astounding) card art.
- Amazing voice acting.
- Despite a lot of mechanics Legions is pretty straight forward and relatively easy to get into.
- Devs are fairly active and this game honestly came a long way from what it used to be.
- The client is super light and can run on a potato.

- You can't craft an entire deck from scratch on demand.
You have to grind your way towards top decks hoping that the needed cards will drop in a crate, farming dust on dupes, and checking the shop that rotates every day.

- Everguild is obviously smaller than Blizzard, so content delivery speed and its amount is less than in HS.
That means less modes, features, events, community interactions and generally things to do in-game.

- The prices for premium battle pass and special offers are too high.
The profit you receive from it is not worth the amount of money they ask. I wouldn't mind buying exclusive items, but that price always stops me.

- I consider Legions f2p friendly.
I don't mind slowly building up meta decks. I enjoy just playing the game, being in it and not aiming to win as much games a possible. I had my fun in the lower ranks when I started and I still have fun on higher ranks after years of playing. If the only fun part you find in CCGs is winning, Legions will leave you unsatisfied.

- This game is very board-centric.
You rarely can completely clear the board with 1 spell or a combo from hand. While in HS it happens like every 2 turns.
Legions also doesn't heavily rely on combos (it has little mana cheat and cards are more expensive). Even with prior setup you can play max of 4-5 cards in a row. This adds value to your units on board and makes you care more about them (while in HS they are just cannon fodder). This game relies a lot more on units (their effects, buffs, interactions with each other) rather than dealing damage directly from hand. I like that a lot about Legions and that change in play style definitely makes it different from HS.

- Some say newer expansions ruined this previously well-balanced game.
Power creep is inevitable in all CCGs and if the newer stronger cards wouldn't replace older once, the game would be just on life support with nothing new happening. I'd definitely go with new things even considering the fact that initial balance was funky and many people's favorite decks was no more.

Your Horus Heresy game at home.