I don't have issues in terms of expressiveness, but in terms of performance. Complex/expensive regexs that run quite quickly in sublime take a long time on atom, especially on large documents.

Perhaps this is an abuse of the tool, but I find its convenient to do these sorts of things in a text editor where I can watch my regexs highlight it's selections in real time.

Not the parent, but the JS regex engine is less powerful than what Sublime uses (PCRE?), lacking among other things atomic groups, possessive quantifiers and lookbehind.