It makes a refreshing change to read your question - you seem to be one of the very few people who appreciates just how important it is to get consistent output in terms of flash energy. It is of course just as important to get consistent colour temperature too!
There are several factors that influence both flash energy and colour temperature consistency but the main one is the capacitors - their quantity, their quality and their specification. The problem is that AFAIK the only budget manufacturer to actually give this information is Lencarta...
Most of the budget makes only have 2 capacitors at most, some only have one. With too few capacitors, a potentiometer has to be used to vary the output, and this leads to the inconsistencies you mentioned.
The Lencarta
SmartFlash 200 has 9 capacitors
The Lencarta
ElitePro 300 has 8 capacitors and the 600 has 10 capacitors. All is explained
here.
Other makes? Dunno. Someone told me that The Flash Centre told him, in response to a direct question, that their new D-Lites have 4, but I don't know whether that's true or not.
One very good indication of what's inside a flash head is the recycling time. Basically, if it recycles very quickly then the quality of the flash is likely to be much higher than if it takes an age to recycle.
Another good indication is flash duration (although the most important factor here is the length of the flash tube).
As you may have worked out from reading this far, this is something I feel strongly about. I think it's vitally important for commercial photographers like myself to have very, very consistent flash energy and colour temperature, but it's just as important for complete beginners - they need all the help they can get, and if every shot is different then they have no chance of ever finding out why their shots don't work as expected.
) Thanks for your input chaps