UTF8 encodings are defined for ByteString only as that would not make much sense for Text

>>> encodeFAll . toEncoding () $ "\xc3\xb1" :: Either EncodeEx (Enc '["r-UTF8"] () B.ByteString)
Right (UnsafeMkEnc Proxy () "\195\177")

>>> encodeFAll . toEncoding () $ "\xc3\x28" :: Either EncodeEx (Enc '["r-UTF8"] () B.ByteString)
Left (EncodeEx "r-UTF8" (Cannot decode byte '\xc3': ...

Following test uses verEncoding helper that checks that bytes are encoded as Right iff they are valid UTF8 bytes

>>> :{
quickCheck $ \(b :: B.ByteString) -> verEncoding b $ fmap (
         fromEncoding 
         . decodeAll @'["r-UTF8"]
         ) . encodeFAll @'["r-UTF8"] @(Either EncodeEx)
         . toEncoding () $ b
:}
+++ OK, passed 100 tests.