I'd say it's much more difficult to mess up in FE than it is not to. Certain characters are just ridiculously overpowered and, if in trouble, you can just 1-man entire missions. The real challenge is in not killing everything with those so that you can share exp between all your party!
Regarding Three Houses, I must say I expected the overinflated reviews and have been disappointedly... not disappointed. The gameplay is the same as usual, fairly simple and easy no matter which difficulty you choose (which makes it great for newcomers and more casual players), but a bit of a let down for people looking for a tactics game. My main complain, however, is in the story. While the characters are a mixed bag of great designs with touching backstories against walking archetypes with the especial skill "Bore you to Death", what I've seen of the story as of now has been a, if you'll forgive the expression, complete shitshow. The plot is a mess, the monthly timetable makes the pacing feel very unrealistic, the villains are just that, villains... Surprisingly enough, the game actually kicks itself in the balls and self-denies some opportunities for great moments, too.
I do wonder if, by realizing that the casual modern FE is a really easy money cow, Nintendo has just completely given up on creating compelling stories for the franchise.
Now, this all sounds super negative, but really, I'm just focusing on my grievances. If you play Three Houses expecting "another FE", you'll be pleasantly not surprised and have a good time with the waifus/husbandos. If you expect a JRPG with some meat, you'll go starving.