As an agency owner, we don't use these tools regularly but we have clients who do, and we support them. As far as the domains, they are a registrar like any other. You can transfer the domains away or to them like any other registrar. This means that the customer "owns" it for the period of time it is registered for with an accredited registrar. The web builder companies do not own it on your behalf.

There are no real SEO penalties, but as with any web property, you have to do the work to get all the SEO working as you want.

As far as benefits for developers, give me an open source tool any day that I can improve on, extend, or mess up with sketchy coding. These tools are meant for consumers to build their own sites for the most part. They represent the initial commodification of "get a website". They are more difficult and/or expensive to extend than a tool like WordPress, Laravel, Hugo, etc. And they are walled gardens, which means they are difficult to migrate away from.

Thanks. Yeah, that’s good to know that there’s nothing shady going on with the domains. It does seem very inexpensive when you factor in the domain and hosting, so I was curious if there’s any drawbacks.