Depends on which alu rims. Deep alu rims, say 28mm at least, lose heat quickly to the atmosphere. Box rims not so much, but either will lose heat more quickly than carbon which is not a good conductor of heat.
"the thermal conductivity of the C/C composites remains almost constant at the value of 5.28 ± 0.42 W/mK in the direction perpendicular to the fiber axis."
"The thermal conductivity of aluminum and its alloys, which is 88 to 251 W/m K,"
Getting the heat off the brake track and into the atmosphere is the whole deal.
On our tandem with tubed tires on 28mm deep alu rims, we air brake as best we can while letting the bike run to whatever speed, then brake down hard for the corners. If it's a winding descent, I alternate front and rear brakes. We've blown tires off alu box rims. It's not the puncture risk, it's blowing the tires off that's the issue. I doubt tubeless is any better than tubed in that respect and might very well be worse, as they lack the tube holding the bead in place on the rim. I would never run hookless rims on a rim brake bike. Non-folding tires with steel wire in the beads are also not good as the wire retains heat and can eventually melt the bead on a long descent.
You're not completely out of the woods with discs, as discs can definitely overheat. A riding buddy of mine went off into the blackberries when his rear tandem disc got red hot and melted the pad. The precursor was that his front brake had failed and he was trying to bring the speed down before a sharp corner. There's also the relatively well-known story of the single bike descender who was dragging his disc brakes to hold speed constant until they both overheated and totally failed. Somewhat terrifying. Someone had told him that you can't overheat discs.
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