Risk
As a 3PL, I believe the bottom line on this issue is whether or not you risk contracting with a carrier with a less than perfect record. I try to give the benefit of the doubt and if I question a carrier and the warnings were for something fairly innocuous, and I decide to contract with them anyway, then I accept the risk of potentially becoming a co-defendant if the worst case scenario occurs. That's the crazy way of litigation, especially in the US where the lawyer's refrain is "Sue Everybody!". A good litigation lawyer would likely jump all over a 3PL who selected a carrier described above, regardless of the reasons for the warnings. And, in some US jurisdictions, you cannot recover any costs if successful. Then again, no risk - no reward. The only way to ultimately protect yourself is: don't do any business.