This might have been true some years ago, but I cannot agree with it now. There is a well-written IRCv3 spec, which I was following when implementing my own IRC server. I also tested it with several clients and I found out that core functionality and official extensions are pretty consistent. Bugs that I encountered always were a misinterpretation of the spec either by me, or by the relatively new IRC client that I was testing with, but they are fixed now.

The encoding is almost always UTF-8, I never encountered anything else on any network I used.

Yes, some networks may be stuck in the past, that's unfortunate, but the modern IRC is pretty good. Also custom extensions are not a problem since the capability negotiation was introduced.