Mustache is a fine templating library and liked by a lot of people. However I see these drawbacks:
600 lines of code, 9 contributors for 20 line code problem
It’s therefore difficult to understand completely.
A lot bugs arise from people using Mustache and using {{ }} instead of {{{ }}}. The later format is required if you have data which includes & < and > characters. Not using the later form results in subtle bugs that only show up later.
It’s not bad compared to some templating frameworks but having conditional if/not support is starting down the path of building a bad language.
It has unnecessary features like template caching - which exchanges speed for memory usage and adds complexity - premature optimization