Well yeah; the item is considered by a lot of folks to be overpowered. It doesn't require attunement, and it's only Uncommon.Is there any reason beyond strict RAW not to allow dashing on a broom? I'm not convinced it shouldn't be allowed, but if there were a balance issue I might reconsider.
Without some other reason, RAW alone isn't enough to convince me to tell a player "no" on this one.
I think the information Sword of Spirit has put together about the different flying items, their relative rarities and requirements for attunement or not is pretty interesting. I could get behind the idea that they'd be better balanced if the Broom and Carpet were ruled as having set max speeds like vehicles, as opposed to giving the user a speed they can Dash with.
Obviously you can fluff-justify it either way (Oofta's invocation of Harry Potter putting his head down and "gunning" his broom like a motorcycle is pretty vivid), and Sage Advice has ruled that you CAN Dash with it, but I'm not above a minor house rule for balance purposes and to make the four flying items more distinct in their capabilities.