I recently came across this video doing a pretty good job about talking both the good and the bad of RTT application.
Roof Top Tents aren't all they're cracked up to be - YouTube
I considered an RTT, but ultimately, it won't work for me, and here are my reasons, for what it's worth. It might help someone decide; these downsides were just too much to offset the benefits of it.
1. Flexibility, or lack thereof. This is the biggest reason for me. RTT means I camp where I park. This isn't always a good thing, because where I live there are awesome sites where you can camp, but you can't park your vehicle there due to terrain damage you'd inflict. Would I rather sleep near the road or a few yards down by the creek and a better view? Yeah, the answer is clear.
2. Weight. I know some folks run RTTs on stock racks, but mostly you'll need an aftermarket rack to go along with the RTT. The weights will vary, but it's not unreasonable to expect to add a couple hundred pounds, depending on the size and construction of the RTTs that I looked at. In the end, I was not comfortable wheeling with that much weight that high above center of gravity. It just reduces stability both on-road and off.
3. Setting up camp and breaking it down. I like to camp and wheel, and it's nice to have a base camp set up, go wheel and get back to camp without having to set up the RTT. Yes, I know it's fast, but there are other considerations: I will leave cooking gear, some clothes, tables and chairs in camp, so with an RTT I would have to pack them up. I feel better about leaving my gear in tent or with tent set up so it doesn't look like someone left the site behind. Might be nonsense, but that's my line of thought. The more weight I leave at camp, the better it is when I wheel.
4. Getting big dogs up and down the ladder. At night. At dawn. In the rain or snow. No, thanks.
5. Last and not least -- cost. $2-3k for rack and RTT vs. a few high end tents of different sizes and season ratings? I'd rather have my pick of tents, that I can select based on number of people, length of trip, time of year, rather than be limited to the one option. And I'll still have cash leftover for beer, park passes, outdoor gear (clothing, boots, lights, GPS, etc.).