Actually it's the *combination* of factors that's challenging
You've got X Kw (I can't find the FAQ so I've not seen an actual number) Volume and temperature.
And of course the cost
Big items (and big ticket items) have been things like transformers, smoothing capacitors and switching transistors.
Do you need a transformer? Possibly not. Capacitor tech has gotten a lot better (electrolytics are no longer the only choice here) and there are those wide band gap semiconductors.
Personally I'm not convinced the performance improvement they can deliver is worth the cost over a good Silicon power transistor.
The big joker is that temperature limit without liquid cooling. Does that mean no heat pipes to spread the heat? Clever heat sink designs to generate turbulent flow? Hanging curtains of pyrolytic carbon fiber (150W longways)? Forced air? PZ fans?