Oddly, lookbehinds are evil only in a specific backtracking world. We never got around to implementing arbitrary lookarounds in Hyperscan (https://github.com/intel/hyperscan) but if we had done something in the automata world to handle lookaround, lookbehinds are way easier than lookaheads.

To handle a lookbehind, you really only need to occasionally 'AND' together some states (not an operation you would normally do in a standard NFA whether Glushkov or Thompson). To handle lookaheads... well, it gets ugly.