It would have to be an extreme mechanical failure for anything to happen. I also only climb trees that have good bite.
I'm not trying to be a smart $%# and please forgive me if I come off that way but it can and has happened.
I have an API Grand Slam Bowhunter and was on my way down after dark one night when the bottom got tangled in the rope holding the halfs together. I THOUGHT I got a good bite but the rope was between the blade and the tree so the bottom slipped and I was cought by my harness.
If I hadn't been wearing it I might have landed on the bottom half a few feet down but I might not either.
I was still 20 feet or so up so I wouldn't be sitting here today.
Please,if not for yourself then for your family, wear that harness.