A single data point:

I flew Frontier recently to Orlando MCO and successfully brought a carryon (in addition to my personal item) on board without being charged. This was a last-minute trip but a great fare, $40 one way. I decided to chance it. This was a soft-sided canvas shoulder bag - not a roller, but definitely too big to be construed as a personal item. Neither the GA scanning my BP or the other GA standing by the door questioned me. I was able to put the bag in an overhead bin without any trouble. I didn't pop the bubbly until we pushed back.

Like many things, YMMV.