My biggest qualms with the Five Nights at Freddy's series are that, as ingenious as the gameplay is, the scare factor tends to...vary.
Usually my issues with it stems from the jumpscares getting too repetitive and/or looking cheesy. FNaF 2 suffered from that the most. It favored difficulty over horror (though it wasn't without some creepiness. The secret minigames and crackling animatronic screens made certain of that), and the jumpscares looking like gifs pasted over the game background did little in their favor, imo. Bonnie clawing at your face and Foxy leaping at you with his huge jaws hanging wide open could've been a lot freakier if the animations didn't look so cartooney and out of place. FNaF 3 has a similar issue with repetitive jumpscares, since while the Phantoms look nightmarish, they can't actually kill the player, so they can easily attack you multiple times on one night. Put too much emphasis on the jumpscares alone, and the scare factor tends to diminish very quickly.
I do think the series has gradually improved on the scares again starting with the fourth entry, though.