My first reef tank was a traditional 55gal + sump, and years later I "upgraded" to a 60 gal AIO thinking it would simplify my life and reduce number of things that could go wrong.
While it did make the stand much tidier and the tank looked "cleaner", it didn't really make my life easier. I still had the same number of pieces of equipment to deal with (heater, skimmer, carbon reactor, return pump), but now my weekly maintenance took longer since it was all packed into a tighter space and there was less room to work. I also found it less convenient to access since it was at the same level as my DT and I have to work OVER the tank.
I also found my options greatly reduced when I considered adding new toys (bio media, algae scrubber, refugium, etc) - a lot of these meant that I needed to add new HOB hardware or simply not do it all together.
When I then (years later) "upgraded" again - this time to a 130gal DT - I went back to a separate sump. This time, I put the sump in a closet behind the DT so now I have free and open access to everything ... and personally, this approach works better for me.
So at the end of the day I think it's just personal preference - unless you are a really advanced hobbyist who feels comfortable running a tank without a skimmer, reactor, etc (clearly, I don't fall into that camp!)