The problem isn't that sneak level 5 is powerful.
It's that it's more powerful than anything else in the game, by far.
Imagine if getting level 5 telekinesis allowed you to fly above the battlefield, making enemies completely unable to attack you. That would be broken OP, right? That's basically what Sneak 5 is like.
Or imagine if Bartering 5 made all the vendors give you anything you want for free. That would negatively impact the game, because now there's no sense of value to any of the loot. That's what Sneak 5 does to combat.
I would argue that glass cannon is as powerful. A rogue with 8 AP and 5 sneak isn't so terribly broken. A rogue with 16 AP and 5 sneak, on the other hand, yeah it can seem pretty rediculous. I personally created a rogue just to do this and until glass cannon, it's not so great. It would be easy to fix with a cooldown of one sneak per round though, so you can't dump 15 AP to kill a boss before it gets a turn.
