Like all re-sellers they are what they are, beholden to short term market movements. Personally I don't mind them as I don't like to be like my parents and "build a balance for winter". You pay roughly what you need to and deal with the ebb and flow of weather. So long as you aren't in credit there isn't any real issue other than if they go bust you get chucked onto the energy regulators "preferred providers standard tariff". That can be a royal pain in the backside for other reasons due to limits on switching, overpricing, etc but I think is another topic entirely. I'd say, if it's a re seller they are much of a muchness other than price, so in that case if price is the primary factor go with the cheapest, dont build up a credit balance and you'll be fine.
Well it does seam a lot more resellers are popping which I don't really mind some even look good on paper and most of the time you can do most things online or on the app but it all comes down to how good they are when you have a issue.
im with bulb at the moment which is good never had to call them and same with tonik which I was with in the past
recently the only time I had a issue was when I was with GB energy and they went bust.