Walkout basement implies slab, at least for lower level. Effective at heating sure, cost-effective not so much. Especially in areas with any need of AC.

Generally, any radiant floor where you can actually feel the heat, indicates a very inefficient home. Thermostat response delay can make radiant floor heating less comfortable for some users. Most high performance builders are concentrating on the building envelope and sticking with high efficiency heat pumps.

Radiant heat in a slab floor requires specific insulation below the slab. In the long run it would be more cost effective to build the lower level with enough height to install an insulated raised floor above the slab.

It’s never economically feasible to retrofit. IF the house hasn’t been built yet, AND you don’t ever use AC there, AND you don’t mind paying the 40K premium to up the sub slab insulation and add radiant tubing and the boiler, AND you use it for the whole house for heating, then it is very very comfortable. Comfort costs a premium.

"Most high performance builders are concentrating on the building envelope and sticking with high efficiency heat pumps."

Springtime, yes, building techniques are improved and efforts to obtain tightness and better insulation are more common.

But to say that builders are sticking to heat pumps is your local experience and not necessary applicable to many parts of the country. Where natural gas is available and moderately priced, it's a cheaper way to heat a home. Also true where electricity is expensive. Where I live in Northern California is an example - electricity is expensive and gas is always favored for heating. In the areas where natural gas isn't available, people typically choose to use propane. But still not usually heat pumps. Heat pumps are not common in any event.