There's some generic stuff here that may be of help.
http://www.energysavi...-boiler-for-your-home
There's a difference between 'less efficient' and 'inefficient' - it's called a matter of degree!
Virtually all condensing boilers are A rated - which means they convert 90% or more of the input gas/oil into heat - the rest disappearing up the flue. Compare this to the worst G-rated boilers that are the subject of the recent Government-inspired scheme to spend taxpayers' money which are around 60% efficient. It takes a lot of non-optimised working of a condensing boiler for it to fall anywhere near the old G rated boilers.
It is true that boilers are designed for an input temperature rnage and in these exceptional conditions, any boiler will be slightly less efficient. That is not a reason to ignore them.
Your comment about not functioning well is well-made. It was certainly true that when these things first came in about 6 years ago, the early ones had more failures - but it isn't that true of virtually all new technology? Don't be an early adopter is a good rule. But that was 6 years ago, and I think if you ask a few more questions of heating engineers you will find that the early bugs are a thing of the past. They are more complex in design than non-condensing boilers.