FGO fans don't really call the gameplay surprisingly complex. I'd say the team building can be fun once you're in mid to late-game and have built up a decent roster with varied tools and niches, but I wouldn't say that makes it "surprisingly complex", more that that's where the basic fun of the game is.

The free units aren't the best in the game but several are great and the game can 100% be done with just f2p units.

The pic is spot on about the story, though yeah. First arc is rough, but late first arc and most of the second are pretty great. Also yeah, the soundtrack is awesome, though really, I feel like that part is just actually true of most gachas. BA, Arknights, FGO etc. all do have awesome soundtracks.