What is the problem with a 7.5" 5.56?

I've never had an interest in a 7.5" upper until recently. Mostly because of the well rehearsed hivemind talking points about velocity, and terminal performance.

But after looking into it, I don't really see where it all comes from. It seems like the ballistics data puts even a 7.5" 5.56 upper well above a 9mm one.

Obviously it's not for long distance engagements. But for like a bedside fun, or one you can stow in a bag in the backseat if you feel the need seems kind of perfect.

Am I missing something here? Is there a reliability problem? Something else?

To preempt some of the things I already know are bound to come up:

• Yes I know a 10.5 is better ballistically, that's why I have that and not a 7.5, but it isn't as compact. Just trying to explore this specific tool here.

• No, I'm not interested in 300 blk. I want all my rifles to run on the same mag, and the same round.