the problem really comes into play when you have an invincible ppu clipping through people, killing a normal person is hard when they clip like crazy, a ppu well that just takes it up to the realms of invincible. Again this is hard to balance cos in no way am I saying ppus should be easy to kill, but the current level of hard to kill when combined with clipping can make a competent ppus almost impossible for a full OP team to kill unless you literally post a anti buffing ppu at spots around the OP so you will have somebody with a lock on for the anti buff 100% of the time and pistol spys / apus chasing them who can keep up while shooting (a tank wont do cos they just can't keep up with a clipping ppu so wont be hitting enough).
Hopefully the big balance thats coming will sort these issues a bit, I personally don't have a clue how they will do it but I hope they think up something. They seem to have done well so far lets hope they keep it up and surprise us with some genius method =)