I can't speak to the experience of a online 6-pack class but I can for the in person, classroom offering from Sea School. I very much enjoyed the class and time spent with the instructor as well as the other students, honestly it was probably the most enjoyable part of the class. Everything mentioned above is true, there are a lot of questions on the CG test that an experienced recreational boater is just unlikely to know including the personnel deck qualifications (ratings) for big commercial. A structured classroom environment will make it much easier than sitting for the test, you also take a less extensive test than if you just sit for the CG test directly, the combination of classroom learning allows the schools to offer they own version of the test and you are not going to encounter any topics that the school did not cover.
I didn't see it mentioned, but logging your time and experience is one of the biggest hurtles for most as you need 365 days within 5 years. You can vouch for yourself if you own a boat but otherwise you need the vessel owner or operator to sign off on this. I suspect a lot of boaters fudge the numbers on this as it is a hard number to acquire when you are a seasonal, recreational boater. I was working on charter full time and it was still hard to come up with that many hours. Another hurtle for many aging boaters can be the physical, a transportation physical similar to a commercial truck driver is required, some older boaters with pre-existing conditions can find this to be a challenge depending on the physician doing the exam.
The renewal process is every 5 years and can be a bit of a pain in my opinion, I let mine lapse after renewing it once as I was not working full time in a marine industry and between the physical and logging hours, it just didn't seem worth it. Jurisdictions (states) or insurance that requires boating safety courses will accept an expired USCG license as an acceptable qualification. Most trawler owners tend to be old enough to be grandfathered in and not required these boater safety classes anyway.
The following is just here-say but sounds possible to me: A licensed captain is scrutinized to a greater extent if an accident occurs. I would be nervous about holding a license based on sketchy time logs in the case of an incident. I suspect it is the only time your experience would be scrutinized and perhaps creates an "out" for your insurance that wouldn't exist for a non-licensed captain.