My take is that a combi is an ideal system for a small flat with just one or two people in it, where it can cope with occasional hot water demand well, and doesn't have a lot of CH to do. The lack of hot water and header tanks saves space, complexity and cost.
In a bigger house, pressurised stored hot water beats it hands down. You have the space and likely as not if you have two bathrooms, it's because you are going to be using them simultaneously.