UTF-8 is a way to write Unicode in a backwards-compatible way on media that support 8bit per character tokens. It is a variable length format which uses between 1 octet (for ANSI characters) to 6 octets. Most languages, except the really exotic ones can usually be represented with sequences of 1-2 octets per character.
Unicode is a family of standards (I know at least two different standards) which represent characters in words of 16 bits or 32 bits. Maybe there are even more standards which I do not know about, but that does not matter. The characters that UTF-8 encodes are really words of 16 or 32 bits.
If you are to represent Unicode text in a wxString, this is done by using wchar_t characters. On Windows, these are 16 bits, on my Linux box, these are 32 bits. Whatever size it is, sizeof(wchar_t) != sizeof(char), because if you pass "ABC" then you do not really pass 0x41, 0x42, 0x43 -- in reality, you pass two (four) times as much data, so for example 0x41, 0x00, 0x42, 0x00, 0x43, 0x00. (In fact I have no idea about the actual encoding -- what matters though, is that these are 16/32 bit values).
So obviously it cannot work reliably if you hand this data to some library which expects characters to be octets. It may work for a while, and then fail randomly due to a thing as simple as calling strlen() on a character string that happens to have 0x00 as the upper byte somewhere.