But that doesn't have vias...
OH, you meant a comma there, the FR4 with vias, versus plain aluminum core? I see!
You can run the numbers, take typical footprint dimensions and combine with the specs of the adhesive layer to find thermal resistance.
The thermal spreading of aluminum is obviously far higher than FR4. You can apply the same steps (take typical via dimensions, copper plating thickness, copper conductivity, etc.) and derive the values for FR4. It's rather unimpressive, which is why we use metal core for things that need it, like lamps.
Additional routing layers tend to be difficult to fab (expensive) so metal core PCBs aren't used so much for power electronics; a single routing layer isn't enough to realize most designs. But it's more than adequate for simple designs like chains of LEDs.
Tim