when a new bad guy shows up juggernaut seems to develop some huge weakness, or his power level drops considerably so that the new guy can take him out easily and impress everyone enough that they buy more comics.
the onslaught thing was completely stupid. not only was the gem never in the juggernaut, but unless onslaught possesses more physical strength in his thumb and forefinger than there is in thors godforce, it shouldnt have done anything to him. juggernaut has withstood thors godforce and not gone down, he was prepairing to walk into it to get to thor before the ground caved in. in the onslaught saga psylock was shown to repeatedly read cains mind while he had his helmet on, which is also impossible going by any other comic ever written about the juggernaut. its the focus point of most issues hes in, but it was disregarded.
nimrod taking him down with sonics was just about as stupid as the whole onslaught thing was.
spoilers: anyway, in case anyone hasnt read it yet, this fight actually occured in the latest x-men wwh comic. juggernaut got his full power back and during the exchange they had both hulk and juggernaut seemed to agree that pre wwh, juggernaut was more powerful, but hulk said everything that happened before doesnt matter now. so they struggled and neither could force the other back, so hulk didnt have any kind of physical advantage. then professor x distracted cain, and when he looked away the hulk took him out via bfr. so going by that wwh doest really have much of an advantage over the juggernaut, the the fight was "won" because juggernaut was distracted and hulk sidestepped him. then the juggernaut landed in some swamp and hulk said by the time he gets out hulk will be gone anyway.
it would be a cool fight to see, but it looks like wwh doesnt have any advantage going into the fight as far as strength goes.