I always take a small carryon luggage (bag with wheels) into WDW parks and had never had a problem until last fall.
First, at Magic Kingdom, when we went through entrance the magic band scan, a woman told us that we couldn't take the luggage into MK. So I told her that I would take care of it. The she said OK and let us go. That's it! Seriously!
I later looked up WDW rules and found out that my luggage was totally allowed because the rules says luggage must be under 24 inches, and mine was only 18 inches.
Rules:
https://disneyworld.disney.go.com/park-rules/
Then, at Blizzard Beach, I was again told that my carryon was not allowed and must be put into storage. So I was pulled aside and given paperwork to fill out. (At that point, I still hadn't read the rules.) But before I even started filling, a manager looking person walked over and asked what was the problem. After I told her, she said that wasn't true, I could totally bring the luggage in. Then she proceeded to reprimand the worker who held me up, so I high tailed out of there!
What I learned: study the WDW rules before you go, and put it on the cell phone for ready retrieval so that if the CM comes up with some nonsense, you can readily refute them. Also, demand to speak to the manager. The CMs make minimum wage and doesn't have a clue about nothing.
Side note: I've never had trouble taking my carryon on the buses.