I'm sure it works quite well with UTF-8. I'm less sure that it is able to consistently distinguish e.g. various Cyrillic encodings. And yes, CP1251 and even KOI8-R are still occasionally a thing when it comes to websites.
Have you encountered concrete cases of failures with Cyrillic single-byte encodings (where the letters fail; there are cases where box drawing characters fail due to preferring letters over box drawing)?