Are you saying mainstream torrent clients don't check the hash? As far as I know, not only do they, but they ban peers who have sent them wrong data more than once. So you could DoS them for a bit with lots of peers sending bad data, but you need a lot of ips to do that because you'll quickly get all of them banned. And unless you are doing this through residential proxies, people will learn your ranges and block you by default.

Maybe there's a DoS you could do with uTP by spoofing someone else's IP and getting them to ban a real peer, but you'd presumably have to get in between them requesting blocks and reply with bad ones, which realistically means you are a MitM, so you could DoS them more directly by just dropping their traffic.

Or if you mean more generally that a malicious packet could reach a client and exploit a memory bug or something, that applies to literally anything on a network.

Suppose you have a torrent client that saves chunks to the filesystem before performing integrity checks. Suppose also that you have an antivirus program that scans every newly-created file for malware… and someone sent you 42.zip. Sure, the torrent client will reject it later, but the damage has already been done.

This specific scenario is unlikely (most antivirus programs can cope with zip bombs, these days), but computers are complex. Other edge-cases might exist. Torrenting is safer than downloading something from your average modern website, but in practice it's nowhere as safe as the theoretical limit.