I have used btcrecover several times to help others crack forgotten passwords or incomplete seed phrases. I have never experienced any problems whatsoever. Additionally, it is all open source, so you can examine the code yourself to see exactly what it is doing.

Regardless, I would still recommend installing and running it on an offline machine, if you can. Not because I have any concerns about btcrecover, but because you don't know what other malware you might have lurking on your computer which is waiting to steal your bitcoin if you regain access to your wallet.