Once an account is already vouched, it will likely face far less scrutiny on future contributions — which could actually make it easier for bad actors to slip in malware or low-quality patches under the guise of trust.
You personally might stay careful, but the whole point of vouching systems is to reduce review effort in aggregate. If they don't change behavior, they add complexity without benefi.. and if they do, that's exactly where supply-chain risk comes from.