They look incredibly stressful for the animal, and like they could pose a potential injury risk if the animal struggles too hard. I absolutely do not recommend their use.
Restraining an animal for grooming stuff is fine – if you’ve trained them to engage in it voluntarily beforehand. (Or if there’s a legitimate emergency, but at that point, I’d hope you’re working with a groomer or a vet). Shoving an animal into a restrictive bag where it has no ability to defend itself or even really change position is not. You’d probably get your cat into it once, ever, and then the moment he sees the thing you’d never be able to get near him because that would be such an unpleasant experience.
Honestly, in your situation, I’d just work on training him to allow nail trims (there should be posts on that in the blog archive, but if not, just google it – there’s lots of articles on using r+ training for nail care). I’d talk to your vet about the flea issues – I’m pretty sure you don’t need to bathe a cat to kill fleas now that we have chemical treatments and preventative care, so hopefully you can just address the issue that way and not have to put your cat through baths.
Our cats are indoor/outdoor (same here, parents), and we use flea treatment on them. Monthly treatment, a little dab of stuff on the back of the neck, alternating brands on the advice of a vet who said it would help prevent the fleas building up an immunity. If we’re on time with the treatments, no fleas. If your cat will keep a collar on, that can also work.
Go ask a vet about treatment. Don’t bathe your cat, it’s bad for them. Bathing strips the oils out of their fur and dries out their skin. Plus, for most cats it’s incredibly stressful, and restraint would make it worse.
You can absolutely train him to make it easier to trim his nails. Gradually train him to tolerate having his feet handled by rewarding him with treats as long as he tolerates it, then work up to trimming claws. Or give him better scratching toys indoors to let him wear the claws down himself, our cats don’t need their claws trimmed at all because they scratch on trees and the like.