Are they acceptable for fairly simple cases, which mostly means patients who had braces growing up, didn't wear retainers, and don't need much correction? Maybe, even probably.
If you're considering D2C, it's worth visiting a few local orthodontists for in-person consultations (which are generally free) and openly discussing the pros and cons with them. You'll probably find at least one orthodonist who will either say "Your case is so simple that, while I'd do a better job, the odds of D2C going terribly are pretty low" or, well, anything other than that. You can't lose from having the conversations, and one of them may earn your trust and present something that works for you.
D2C vendors' biggest weakness has been accepting too-complex cases and/or accepting patients without having an unbiased orthodontist review the case first. Try to fill that gap yourself and then see what you think.
IOW: It's not a scam, but it is a higher-risk, lower-quality service. It isn't a cheaper version of an orthodonist. If your case is simple, money is tight, and you know what you're buying, those may not dealbreakers.
(You're probably already beyond it, but for anyone else: https://www.youtube.com/watch?v=Ogbo9Ny_y-g and https://investors.smiledirectclub.com/news-releases/news-rel... are decent starting points.)
I don't use a product like this but if I did I would get my dentist to set me or my kids up with Invisalign. My understanding (and it could be wrong) is that they are basically the same thing only professional rather than DIY.
Of course, if my dentist said my kids needed more traditional braces I would go that route. I try to follow the recommendation of professionals I trust.