This is just my opinion obviously, as I have been in all the schemes at some point except Stroma.
I was in the NIC for a good many years, even before you had to to be by law, I left them just over 5 years ago after a straw the broke the camels back moment. The feeling I always had with NIC was that THEY were doing you a favour, that you had to jump when they said and it was always a feeling of jumping through hoops for them.
They always reminded me of the old BT attitude where they thought they were God, until competition came on the scene, and even the it took time.
Then there is NAPIT, best way I could describe them is this.. I am the type of electrician that will always use a good quality MK double socket or light switch and not a cheap made in China one that costs pennies, even if it may do the job, hope that explains, Nuff said.
Then there is ECA/Elecsa.. been with these Guys for a few years now. You are the customer and THEIR Boss, that's the karma they give off. But you still need to be on top of your game, but never a feeling of they are out to get you, Even the yearly inspection is a joy, sadly though the guy I had in Cambridge has now retired.. Thinking of anything bad to say about them, but can't, A joy.