The major downsides are that the exposed wire is much less corrosion resistant underground (which is irrelevant) and that you need appropriate lugs. But standard lugs can easily be purchased in aluminum-compatible variants at no additional cost.
There was an issue with aluminum wire and terminations, the wiring devices and breakers used for copper only wire didn’t work with aluminum (wrong coefficient of expansion of the metal where the terminations were made) and caused arcing and fires. Once the termination points were made out of an alloy that worked with both Cu/Al, it’s been safe to use.
I agree with your general point, but be a bit careful here.
> Wiring devices and breakers have accepted both Cu/Al wire for the last 50 years.
It’s true that, for a little over 50 years, the UL has regulated what devices may “accept” Al wire. But devices have had markings suggesting use of Al wire for longer than that, and it was quite unwise:
https://inspectapedia.com/aluminum/Aluminum_Wiring_Repair_CO...
Don’t get caught up in all the discussion of repair devices — AIUI this article is mostly about “repair” of old pre-8000-series Al wire. You can’t buy that stuff any more. As far as I know, modern Al listed building wire is only sold in 8 AWG and up, and this stuff doesn’t need “repair”: it’s perfectly fine in any appropriately listed connector.
I was taught that the safe way to keep using the old pre-8000 series wire is to use the specific listed fittings for this purpose (WAGO 221 series nuts are fit for purpose) with contact paste and a copper pigtail that connects to the device.
The cost of copper for house wiring is not significant (if it were, every spec home builder would be wiring houses with #12 gauge Al instead of #14 Cu) until you get into thicker gauge stuff, and that's almost always Al anyway.