This whole concept of "mental stability" is very interesting, i mean, can we consider that Jake was stable enough for example ? because even after being 1 evolution he still was so meek that Ella had to literally beat him up trying to make him use his powers, he only got serious after fucking up Mia, in that case he was at least as much as a pushover as Emily.
Or Claudia who is a total sweetheart, i don't think she is more assertive than Emily, stable maybe, but does it mean that the person needs to, well not have any mental issues and Emily might have some ?
Before Ella said that i imagined that only being phisically compatible was enough, now there is a lot more to consider.