A lot of past Nintendo consoles have made pretty major changes to the hardware and operating system compared to other console manufacturers, which makes backwards compatibility less likely. The Wii U basically booted into the Wii operating system to play those games. I imagine getting Wii U games running on the Switch wouldn’t be the easiest thing to do, though certainly not impossible. We don’t know a ton about what changes the Switch successor will make, so it wasn’t outside the realm of possibility that it would be the same.
IMO they’re both good options. Afaik, Tuta has an open source backend, unlike Proton. However, Proton has had great recent independent security audits, and court orders against them have shown how little information they keep.
Proton offers more features and services, which also makes it a bit more expensive. I’ve used and like both, but right now I have a Proton Unlimited subscription because I like their drive, vpn, and password manager.