HE currently do not have schema for Oberon (maybe need to migrate one from UE or TextPad if somebody need). So, I can not compare. Probably, if language has very complicated syntax, that can not be covered by regexp definitions, and has dedicated, hardcoded lexer for scintilla, scintilla will be better (actually not scintilla as you mentioned, but lexer). But is it cost effort.. ?
C++ in Scintilla is not more powerful (maybe they can parse #ifdef _DEBUG, HE cant ). But HE can highlight special characters in strings (like \n \r \t etc), highlight functions and classes, todos etc.
So, it it rather time consuming to create for HE Scintilla based lexer + Scintilla lexer will not provide all information HE needs.
If somebody else would like to help with such plugin, I will provide all needed help

BR, Alex.