I think a lot of the problem is that when they first came in the courses were more intense, more expensive, gave better quals and better understanding. It was more 'niche' than it is now.
Plus CPD had to be done with recognisable quals etc so DEAs kept up with new initiatives and energy/cost savings measures. EPCs were supposed to be a part of the HIPS and would have added to it. Keeping them on once HIPS went devalued them.
When DH first started he charged 80-100 for an EPC. It took a good hour in the house and 2 more on the system. The programme has changed beyond belief and many of the questions have disappeared. What used to take a couple of hours now takes half of that.
Courses are now a fifth of the cost and can be done completed within a month with no on the job experience and audit requirements are minor.
DH has had to drop his charges to compete with the 'national' companies that are offering EPCs for 35 and paying the DEA 17. People aren't taking the care they used to and the home buyer doesn't care what it says, they just want it cheap. So the DEA doesn't bother to use the addendums or take out the wind turbines and instead of blaming the buyer who just wants it cheap they blame the DEA for being crap - buy cheap buy twice as the old saying goes.
The 'idea' of them was sound, but they have been devalued by people that didn't understand or didn't care what they said. I think there is still a place for them, but only if they are completed properly.
Like I said before, the things to look at are the things you can change yourself - boiler, loft insulation, DG, secondary insulations etc. PV is a good idea if you can afford it - buying it outright means you will see savings almost immediately. We put it on my PILs home when the FiT was high (9 years ago) and she has a rebate every year that almost cancels out her heating bills
. We will be doing the same to our 'forever' house this year.