I really like troff and how much lighter it is, I just wish it had proper Unicode support for obscure Asian scripts. I haven't tried latex's Unicode support, but I've used Sile and it requires some manual hacking.
Unicode support is perfect out-of-the-box these days, in the sense of supporting arbitrary UTF-8 input and outputting the corresponding characters. Some of the "complex" scripts (Devanagari, Arabic, Thai, etc.) have a few issues, but all the common ones should work fine with the appropriate packages.
XeLaTeX and LuaHBLaTeX are both HarfBuzz-based, so if HarfBuzz handles Khmer properly, then you'll likely be able to get it to work with LaTeX, but it will probably need a few tweaks.
Unicode support is perfect out-of-the-box these days, in the sense of supporting arbitrary UTF-8 input and outputting the corresponding characters. Some of the "complex" scripts (Devanagari, Arabic, Thai, etc.) have a few issues, but all the common ones should work fine with the appropriate packages.