Is it cancer on smaller maps? Yes. Definitely.
Is it OP? No.
Being cancer does not make it OP. Lots of things are cancerous. 222 spam vs USF on map like Steppes in 3v3. Especially if you're fighting up top. Wide open space, no cover, no angles. USF stands no chance. Or maybe kubel spam on the same map. Also no chance for USF. Path spam with scotts is quite cancerous to play against. Still not OP. Neither a 222 spam on large open maps, neither a kubel spam, neither a paths spam. What else is cancerous? IS 152 on lane maps. Not OP. Cancer. Solution is clear, dont blob and plan attacks or react with an elefant/jagd. Same jagd and elefants are cancer on maps like redball. Perfect 3 lanes to nullify the lack of turret. Or maybe on redball the perfect undestructible sight/shot blocker on mid that the Sturmtiger can lob a nuke over? Or maybe as OKW going vs a maxim spam on lane-y maps. Or as USF going against am MG42 spam in buildings? Or maybeeeee. Play the Hamburger Port on top spawn... AND fighting for the bottom fuel. Worst map design ever. More cancer? Arranged team where one player is spamming calliopes and ranger zooks. Not OP but a dick move nontheless.
Got any more cancer BUT not OP scenarios? Anyone?
Pak43 on some maps is cancer. On [most] others a useless shitstain.
Feel free to add any more cancer stuff.
Point is... Sure. 120mm is cancerous but that's due to some maps rather than the unit itself. Problem is that you need to invest more nerves in dealing with it and hence the perception of OPness. On small 1v1 maps it can be camped outside, close to base and cover lots of middle ground....like Leig and pak howi... Of course, pak howi has a narrow firing arc, hence it needs to reposition itself often, losing a lot of "shelling time". 120mm has a great arc so it can continuously fire. If you further nerf it, it becomes useless in teamgames. In a 1v1 its probably best to invest in one werfer/stuka and try to sneak in one squad to finish it off if the rockets dont.