Well that depends.
I think that you can always implement this safely for types without padding bytes (e.g. repr(packed), or maybe even repr(C)?), and also for all types if the implementation does not touch the padding bytes in which case the size of the [u8] might be smaller than the size of T.
If you are talking about implementing FromBits<T> for [u8] by memcpying all the bytes for types with Rust layout including padding bytes then, as @hanna-kruppe says, that will depend on whether reading a padding byte is a read from uninitialized memory or not (cc @strega-nil @RalfJung).
In any case, given that the layout of types with Rust layout is unspecified (e.g. the compiler can reorder fields at will), you will run into issues when serializing/deserializing with different Rust versions on top of the endianness issues that you get when serializing/deserializing on different architectures.
At that point you might as well restrict that blanket impl to repr(C) types and call it a day.