They are fun but a compromise. My son slept in a roof rack tent on a US holiday. He thought it was epic. We preferred life in the (very simple) van underneath.
We did think they weren't a good UK choice due to many models not having good waterproofing from the side, but models like the one you link to seem to overcome that and I do seem them around now and then. If someone sleeps in the vehicle, any movement in the tent will make them feel like they're rolling about in a boat. You don't notice that up on top. You're also now a ladder away from a wee in the night unless you're a good aim into the pee bottle. There's usually no porch if you need to get into it in the pouring rain (they are adapted from safari tents, designed for good weather and the odd shower), although I've seen extra sides for the underneath so you get a sort of downstairs porch. You need to go to ground level to cook or for a brew unless you can safely balance a stove on the roof. You need to empty and collapse it to drive anywhere during the day.
OTOH, you get a nice view and lots of conversations at the campsite. The foam mat in our rented one was quite comfy.
The issue in my view is your permissible roof load, which is the combination of rack mass + tent mass (+ any other roof load). If adding the tent exceeds that, you might find that your insurers will tell you where to go in the event of an accident or damage. Plus, I suppose, you have the issue of any potential damage to your roof rails once you're in it. So it depends on your vehicle. Check the owner's manual and your roofrack specs.
I have a transit custom. Max roof load is 130kg. Max rack load is 80kg with the two bars I have. Hence I can only weigh 30kg! More bars would add more load, so yes, they need beefed up.
Having tried both, we opted for losing one car and converting a van into a day van/camper. Both so we don't overload a car roof, and because van life was worth the extra.
