They are fine provided they are specified with adequate capacity. Because they heat the DHW as required, they have to have enough capacity to be able to do this AND run the CH on the coldest days of the year. More questions on AB on these boilers seem to crop up now (when folks are running their CH) and not at other times of the year (when they still running the DHW). The 'problem' is perhaps that too many plumbers are not familar enough with estimating the total energy requirements of the house - or that folks put bigger and better power showers (non-electric types) and just expect the boiler to cope.