>6m and 10m have extremely variable performance
You got that right. But for long distance.
They have very sporadic long distance performance. On summer days, the E-layer of the ionosphere well, ionizes, and then skips are possible. I think the farthest I got on 6m was Enid, Oklahoma, from basically Casa Grande, AZ.
Same goes for 10m. Farthest was North Dakota on E-layer.
However, despite 10m being HF, they both act like regular VHF. Good for local ground-wave stuff, and plenty stable. I could reliably hear the 'local' 10m repeater, about 50 miles away.
I have a feeling the use of these bands dates back to WWII, when the upper end of the VHF spectrum wasn't all that practical for good cheap day-to-day comms.