The type of energy source is irrelevant. Stone flooring takes too long to heat up and too long to cool down, and any other sort of flooring is mad.
Ahh, you've experienced a typically badly setup system then
UFH got a really bad rep back in the 70s and 80s (IIRC, I recall one of my grandmothers had it in her flat). This was typically electric, and got a really bad reputation because people tried to use it badly.
2 ways :
1) They'd try and use off-peak lecky and the floor slab as a store. So you get cooked feet in the morning and it's gone cold by the evening when you want it.
2) You'd turn it on only "when needed" - leading to the observation you've noticed, that large slabs do take time to warm up and cool down.
But the latter also applies to a system with radiators. If you let the house go cold, it takes a long time to heat up the building again. Yes,
some of the air gets warm very quickly, but the building takes a lot longer (and in the meantime, your feet are cold).
My brother has put UFH in his cottage renovation. I have to say, his place is more comfortable than ours - I generally find the issue to be cold feet which is a design feature of "traditional" heating methods. Yes, extra socks etc are one answer - but then the reductio ad absurdum to that is we don't need any heating at all since we could just wear more/thicker clothes all the time (which is of course what used to happen a lot before central heating etc).
You do need to setup the system right, and also think ahead - ie not get used to making big changes to the stat and expecting instant results. But it's a darn sight better than the utterly c**p night storage heaters we have in the office at work where you need a crystal ball to work out what heat you'll need tomorrow to set the input control today