I had problems with YaCy. Slow search and slow crawling. Regarding search I think it could be improved if instead of HTTP requests to other peers a more efficient protocol (UDP) could be used. Regarding crawling it's quite possible that doing this in Java and running it on a Rock64 may not be the best combination (: It started OOMing after some days.

YaCy's results weren't great and there was the raking was very bad letting sides game it by just spamming a ton of keywords.