On the downside, even today, Tcl can't handle characters outside the basic multi-lingual plane. It only does UCS-2, it can't handle UTF16 surrogate pairs. If you convert an astral-plane codepoint, such as some popular emoji, from UTF8, TCL will convert each UTF8 byte into a separate unicode codepoint. There are similar catches all over, it's just not practical to deal with non-BMP codepoints in TCL, even just to round-trip them.
Rereading OP's comment, I misread it a bit and responded to the wrong part. You're absolutely right, progress on Unicode in Tcl stalled out after the low-hanging fruit of UCS-2 was achieved.
