Here's the thing about the bat mixups, when talking about whether a character is "broken" or not, you need to completely take out execution. This means on both sides. No matter how difficult it is to execute or block, at the end of the day someone is going to get perfect at both.
Therefore when you look at it this way, is it any different to a 50/50? Overhead then low or low, overhead. Are all safe 50/50s broken? I personally believe they're complete ballshit, but in the context of MKX where so many characters can make they're 50/50s safe and plus I don't think it's too much of a big deal.
How Quan works is if he touches you once you're probably dead. This is why you need to counterpick him with a character with strong oki, if you do Quan's in the exact same position as you. If you can't block the vortex it's basically who gets the first hit wins, if you can block it then it comes down to basically a 50/50, which you can do to Quan too.
Verdict: Quan is not OP or "broken", he has very defined strengths, weaknesses and good and bad matchups just like every other character.
Edit: Forgot to add this, if you're gonna do anything then just remove the pseudo-unblockables, that's it. Increase the in-game unblockable protection to cover 5 frames or something instead of just 1 frame, this would also stop Pred and any other silly situations future DLC characters have.