Been running this guy on my rain team and I'll have to agree with you. I prefer it to the bulk up set, who is horribly walled by volcorona. And more commonly, he's walled by Gliscor if you're carrying drain punch.
He desperately needs pain split. In a sandstorm dominated environment, leftovers won't exactly be enough to keep you alive. And he has trouble with some of the top sweepers and taunt+SD Gliscor. He's really bulky and can take hits very well, but I'm not exactly convinced by him stopping things that are already easy to stop, such as Conkeldurr and Toxicroak.
Depends on which bulk up set you are referring to.
The classic bulk up,sub,sucker punch,drain punch set is walled by volcarona and setup on surely.
But if you are a little bit smarter and instead of sucker punch you put a real attacking move to toxicroak then the scenario changes.
I mean surely sucker punch is a very god priority move but:1.it is not good on a setup sweeper that wants coverage,2.it is not good on a setup sweeper that needs many turns to do it's job 'cause it has very little pp.
So if instead of sucker punch you put poison jab you accomplish the following:
You beat faster ghosts with substitute and slower ghosts with non attacking moves that can outstall you.
You hit many of the fliers and psychics that trouble you for very good damage.
You have a reliable attacking move that doesn't have 8 pps and has greater power than sucker punch.
And the downafalls of using poison jab instead of sucker punch are very few.Let's see them:
You cannot ohko Lati@s at +1.
You lose the priority of sucker punch.
Gliscor beats you but he is beating you with sucker punch also so no difference here.
Lati@s get hit by poison jab for good damage so again it is not like they wall you(assuming you sub up in the switch in which is almost always the right thing to do).
The only real disadvantage is that you lose the nice priority when you sometimes need it for faster foes.
Also with paralyze support Toxicroak in the rain can beat many of its usual counters like Jirachi,Reuniclus,Lati@s,Celebi and many more(with Poison Jab of 'course).
Finally Bulk Up Toxicroak with Poison Jab under rain,and with paralyze support if possible,is almost unstoppable.The only 3 surefire counters are Skarmory,Gliscor and Hippowdon.
So no Toxicroak is not as easy to wall or kill as you think...