lurks in the shadows
While Scizor cannot switch in on Rampardos,it can force it out after it kills something so you have to setup TR again if you want Rampardos to kill anything again.Zweilous doesn't really get much better coverage in OU. Either way it's the Steel types giving you trouble. Also, Rampardos has access to better coverage options. Finally, sure CB Scizor can obliterate it, but it isn't going to like taking a Head Smash on the switch very much. I haven't done the calcs, but I suspect it might even OHKO some variants. As for other priority, Zwelious is just as weak to it. Finally, Zwelious' Outrage is actually much weaker than Rampardos' Head Smash. What Rampardos looses in coverage with Head Smash, it gains in raw power.
Zweilous can continuously kill a weakened team without killing itself or dying from priority.
Also the fact that Zweilous is also weak to Mach Punch is kinda mitigated by the fact that it can OHKO Conk while Rampardos cannot.
Of 'course Conkeldur is not going to switch in on any of those pokes but when you do kill something Conk will be there to ruin Rampardos's day since it will very likely Ohko with Mach Punch(you will already have done like 30% of recoil to urself after killing something) forcing you to switch out and setup TR another time.
But Zweilous after killing something can just sit there,stomach the Mach Punch like a chump(414 Atk vs 176 Def & 348 HP (40 Base Power): 204 - 242 (58.62% - 69.54%))and Ohko back with Outrage.
So it is quite clear that Zweilous doesn't even have half the priority problems as Rampardos does.
Also Zweilous has everyhting it really needs to kill the pokes it needs.
Every steel type gets 2hkoed by Fire Fang and everything else gets 2hkoed by Outrage.
Also if you want to just kill everything with Outrage,you can always use Magnezone to help you trap some annoying Steel types.
But you cannot easily trap Rampardos problems because of many factors(Scizor --> U-Turn,Conkeldurr --> Only Wobbuffet can trap him).
Finally the fact that Rampardos has huge attack doesn't mean it is better than Zweilous.
The fact is that Rampardos's attack is just overkill and not needed in most cases.
Zweilous for example hits 663 Attack with CB and Hustle which is all that he needs.
He 2hkoes every non steel type in the tier iirc and 2hkoes even some steel types with it like S.Defensive Jirachi.
Everything that he cannot 2hko with Outrage gets 2hkoed by his coverage moves or is handled by Magnezone.
The only downside it the confusion BUT:
If Outrage lasts 3 turns then you are golden cause TR ends by that time exactly so you would switch out either way.
If Outrage lasts 2 turns then you can just switch out without having lost like 30%-60% of your life to kill 1-2 pokes.
If we are talking about plain CB TR wall breaker-sweeper Zweilous is better since it does the same things as Rampardos except it doesn't fear priority quite as much and he doesn't kill itself in the process.
Again Hustle is not a con only in Zweilous's side since Ramprdos's main move has the same accuracy issue.
And since both of them will be spamming their main moves 90% of the time this means that in the 90% of the situations they will have the same accuracy problem.
Now if you want a Life Orb TR sweeper things are different since the accuracy issue plays a huge role.
Rampardos can use things like Rock Slide to kill every neutral target and use his coverge moves to eliminate resistors.
Zweilous in the other hand will still have the accuracy issue so they are different in this case and i thing that Rampardos wins this one especially with Encourage.