Urogrim starts with his A3 (2 poisons when nerfed), after that he uses his A1 double hit with a 50% chance for poison. If my math is correct, that is a chance of 50% for one, 25% for two and 25% for no poison from the A1. I think it's not unfair to estimate one poison per attack. So after 3 turns he puts out 4 poisons.
FB needs to set up poison sensitivity first, then attacks for a 100% chance of two poisons. So after 3 turns she can set 4 poisons as well, + sensitivity, what makes the poisons stronger. Urogrim has other, defensive utilities in addition to his poison, but he will do less damage vs. CB. Even if you could argue that he fits overall better to a lot of CB teams, as he can remove the stun with a strong healing skill, the fact that we can argue between him (void epic) and FB (non void rare) shows that the nerf is over the top. Using a void epic as poisoner or a rare should be a no-brainer for the void epic, not a "maybe, if you use the right champs in addition, let's see who is better"- kind of decision.
/had to edit my math alittle bit. To early in the morning to calculate.😴