The sole job of the heat pump is to keep the buffer tank hot (~75°c). Fresh water is heated from the water in the buffer tank via an external, brazed plate heat exchanger to provide potable, hot water. This means there is only a small amount of stagnant water, in the pipes, so there is practically no risk of legionella. Super insulated tanks loose very little heat are simple and readily available, so they make more sense than, say, a phase change material, thermal store.

UFH is typically installed in the slab, which works as a cheap thermal store, but it has some serious disadvantages. It is extremely unresponsive, taking a day or so to bring a house up to a comfortable temperature. It relies heavily on insulation, to be efficient at all. By using a low profile system on top of the slab, or wooden floor, this can mostly be avoided. You can even heat the area quickly by boosting the temperature, since it will cool down quickly too.

This is also nice because it can be used on upper floors, without any need for screed etc. If you already have a reasonably insulated floor, it can be installed on top, only adding a few cm and without the need for digging up your existing floor. A leaking pipe, would not be such a massive job to fix, either. This setup allows you to upgrade in more affordable phases, rather than doing everything at once. If you already have radiators, you could use them instead.

One claimed advantage for UFH is that you can heat using off-peak power. I have no idea if that works in practice.
@goibhniu

@BenAveling Thanks, maybe I'm wrong, but I'm thinking it makes even more sense to charge a buffer tank (and battery) with off-peak power, which you can then use when you need it.