I'm not a Fate expert but from what I've seen I can name some FSN servants if caught in Falling Devils mental attack are fucked
Saber: I don't like Fate Zero since its basically a fuck you to Saber even though in my opinion FSN is already a fuck you to Saber even more so because of Gil Medea and Dark Sakura (not that other good guy characters in FSN are not treated just as bad) anyway on both occasions in FZ, when characters have brought up her mental trauma she quickly falls mentally and doesn't fight back either by not defending herself in the King's Banquet even though by all rights she was right or when she was forced to face one of her former knights which mentally broke her for a bit refusing to fight back before eventually mentally recovering enough to kill him. That's not mentioning she's already pretty mentally broken by FSN and her goal is to literally erase herself from history, so hopefully someone else can do what she failed to do and in exchange become an eternal slave to the CF and doesn't even think she deserves happiness even though she's one of the few characters who has a way to get eternal happiness anytime she wanted to in Avalon.
Heracles: Assuming he's even sane enough to perceive what the hell is happening probably falls to based on how he was corrupted and turned into Alcides in FSF, where he is forced to relive his life if my memory of the manga is correct. However, he's probably too insane for that to happen.
Medea: I don't know about Fates Medea besides FSN and what was revealed in the Turas Realta manga nor do I want to, but from what I've seen of her, she wouldn't handle it well since she's already pretty fucked up in the head thanks to her past, and probably would not take it well being forced to face her trauma again.
Emiya: From what I've seen of him probably doesn't take it well either since his goal in FSN was to erase himself from existence since he hated the situation he has put himself in thanks to his past decisions and hates his literal past self with a passion and tried to kill any chance he gets.