I never used one, but I'd assume they're as good at following the plan as you are (I had read some articles that said some of these lenders operate on voluntary repayment, no fees, no collections, but I don't know how widespread that was), but interest rate arbitrage has to be considered against the effort.
It's much less effort to use my everything credit card and get my 2% discount than to do a 0% apr over four months (or whatever), setup a new account, setup payment at that new account, remember to make the payments, etc and for what? Short term bank interest on most of my purchases would be peanuts, especially since I wasn't getting 2% bank interest when BNPL was at 0%.