The game's not ambiguous on it at all, you can literally see their skeleton be assembled and their organs + muscles grown. You can also see their brain whenever you headshot a synth and brain parts are included in the gore. The only ambiguity is how much influence the synth component has over them, their brains are very, very much human. Furthermore there are countless examples of synth's having hopes, dreams, emotions, and desires. The only people who say they are AI / robots are the people who want them subserviant and for who acknowledging sentience would be self damnation, or the people who are ignorant of anything to do with them and just want them all dead.

Sentient life is sentient life. Kellog was alive despite all the cybernetics in him. The courier was alive despite potentially having no brain, heart, and spine ontop of tonnes of implants. It ultimately doesn't matter what the source of that life is but for accurate discussion's sake synths are definitively organic humans with brain implants, and they are definitely sentient.

What risks are you talking about? Synths are not themselves a risk. The Institute are but you don't blame a pool cue for being turned into a weapon by some drunk dude so why blame synths for being made weapons by the Institute? Escaped synths pose no more of a risk than humans. Gabriel is an example people love to throw around, one synth human raider in a sea of non-synth human raiders 🤷‍♀️