My usual problems are 1) the “generous” F2P model (people claimed Hearthstone was generous, for example) turns out to be actually very limiting with the possible exception of if you’re spending 60+ hours a week playing (I might hit 6). Or there’s a flood of freebies to start and then it rapidly dries up, especially if you’re not into PvP (Eternal, say). 2) heavy, heavy PvP focus such that singleplayer content is usually perfunctory, often doesn’t feed into F2P rewards, and sometimes costs amounts of money that’s way out of keeping with the actual entertainment value thereof (Hearthstone Adventures, for example).
Hex of course had next to no F2P model (and I didn’t care because I had already bought in and was perfectly fine with spending money on a game I loved), but it had the best singleplayer content in the business (even if they didn’t focus on it the way they arguably should have) and didn’t charge for any of it (which they probably should have). I’ll be interested to see if Mythgard hits either of those pitfalls for me.