The thing is, her only truly valuable ability is her Royal Blood, and that’s not even something unique to her.
Her ability to determine classes doesn’t actually give her an edge over other Investigative classes. It doesn’t let her find the MM before N4 unless they claim something other than Investigative (in which case she’s not even the only one that can catch the MM on their lie), and it doesn’t let her prove conclusively whether someone is scum or not, just that they might be lying about their claim or converted (while a Paladin/Sheriff/Observer can figure it out conclusively). And like every other Investigative class, she is affected by Framing.
Her Will o Wisp is interesting and unique, but it has very few charges and doesn’t do “enough” to justify it, since it can trigger off BD classes as well (Knight, someone controlled by Possessor) and doesn’t actually cover all scum classes (the actual Possessor unless they control themselves, Sorcerer). The problem with Will o Wisp is that even if it works, a Paladin/Sheriff/Observer using their limited ability to check a suspicious claim actually finds solid proof of whether the person is evil or not. With the Princess, you find someone attacked as you suspected, and they can still be Knight/Alchemist/BD controlled by Possessor.
The only real ability that gives her an edge over the other Investigative classes is her Royal Blood, which is IMO not enough. I think Princess can be strong in games just because she’s an Investigative class, but if you’d always rather have a better Investigative class other than her, it’s a sign the class could use improvements.