not strictly true as the first condensing boilers appeared 25 years ago, they just never took off due to the extra cost involved, but the advent of the energy efficiency changed all that and now they are the norm.
its not just condensing boilers that are complicated but combi boilers in general, the boiler makers seem intent on always adding as much technology as possible and that means more things to go wrong.
you have to treat a modern boiler as you would a car, get it serviced once a year and be prepared to fork out for parts and repairs now and again.
the big issue with condensing boilers is that the condensate is acidic so the boiler (heat exchanger) actually dissolves itself. to get around this the high end boiler makes such as vaillant and worcester use stainless steel heat exchangers so consider one of those.
i have seen charts when i was on my energy efficiency course showing how long it takes in reduced fuel bills for the boiler to pay for itself, it was that boring ive forgotten it however!