You still need a dump station and the stinky slinky for the grey water, why not just dump the black at the same time / place?
I guess it comes down to personal preference. I have had both and wouldn't go back to a fixed black tank.
The US/Canada are different from Europe and maybe other parts of the planet... while we never had a problem finding a place to dump our black tank when we had our old camper, we always needed to go to dedicated camp grounds that had a dump facility. Especially at the end of every trip we needed to go to the nearest camp ground and pay to dump our tanks.
With the cassette, we just pull up to the home driveway and use our guest toilet. Especially since we do not (need to) use chemicals in the tank - so it is essentially "normal" toilet use, and you can dump it any time in any toilet if you need to.
The last tank of grey water goes to the home garden - no ill effects there yet. The tank holds about 5 days of grey water. If it really does overflow (which it never has), it is easy to get rid of a bucket or two full of grey water and you are good to go another day without having to find a place to dump.
The cassette is usually much cleaner than the fixed tank. You can rinse it after last use at end of trip and put it back into the camper. No smells that would develop over time, no chemicals. You _must_ _not_ leave a partly filled cassette sitting for more than a few days. Just don't do it ;-)