There's not just too heavy, but too much intensity with too much frequency.
What I mean by that, is that if your max yoke is 800lbs for 50 feet or so, and try to do 700lbs or even 650lbs for sets of 50' for 5 sets, you will find that your body simply cannot handle that much stress without something giving.
The yoke, as @T3hPwnisher said, should be trained with very little focus on the weight. If you find you cannot complete the weight designated at a contest for your weight class, then you can strive to maybe squeeze in 1-2 heavy sets of yoke but you have to gauge your body and how it feels. I think we all have a limit on how much heavy yoke we can handle, so find that threshold, work up to it but make sure you do NOT overdo it, and follow it up with tons of light work and fast feet practice.
The yoke is easily the most dangerous strongman event.