As a new hobbyist when I bought it, the AIO just felt simple in comparison to a sump. Truth is, it is simple. The drawback is the rear chambers are small and therefore all the extra equipment you add in the future is required to be small. I made it work with the return pump, 2 heaters, carbon/gfo reactor, anto, and a skimmer.
I like the idea that there is almost no way to have a leak short of damage. Unless you have equipment pumping water to or from the tank, it’s pretty failsafe.
The negative for me was the depth (front to back) of the tank. The footprint of the tank is roughly 48” x 20”. But once you allow for the rear chambers, I had about 15” of display depth. Also cleaning the rear chambers is a tight squeeze.
I recently upgraded to a big 180 gallon tank with sump but I still have the waterbox AIO running as a bubble tip nem tank.