Access points/routers/smart TVs/set top boxes are typically be based on ARM/Linux/Android, and should be supported.
While it's safe to assume that C gets a decent amount of use on every platform, you can't expect all platforms to be as well-supported as the major ones. Undoubtedly, some of those platforms would be listed as low-tier if the C compilers cared to maintain a platform tier list. But a platform being low-tier doesn't mean you shouldn't use that compiler there.
As for trusting Rust or C on niche platforms, C is so full of UB, platform-specific choices, and vendor extensions, that it's hard to ever fully know how well this or that project will work. Rust is much less surprising, if it works at all I expect it to fully work. I'd definitely pick Rust on niche platforms if I have the choice.
While not as safe as Rust, it is definitly much safer than plain C.
And in any case, C++ is always a better option than C regarding safety, when nothing else is available.
Go's runtime can be a showstopper, especially for multi-language projects.
C++ may be better than C, but many people feel that Rust is even better.
Rust is already available in almost all scenarios, there's no need to wait for it.