An invoker is a killing machine from the start of PK, but:

1) You really want to surprise people, and

2) You really want to not tank.

That being said even the best players doing that as lowbies will die sometimes.

IMHO if you can't manage to PK with an invoker low/mid and have it in your mind that you can power to hero and be an engine of destruction you're in for a lot of disappointment. They have a lot more tools at higher level and they can absorb a lot more punishment, but the DR, HP, and saves of your opponents are dramatically higher as well.