Couple of things here. First of all, I agree that it looks very nice but have you ever tried to step into one of these tubs installed like this? It is very awkward since the normal mode of entry should be a step that is equal in height of where you would step into the tub. Stepping down into the tub feels unsafe, especially with nothing to hang on to.
Secondly, as others have said, the service aspect of the tub is lowered unless the pit has at least 3 ft of clearance on the sides where the control panel and pumps reside. If your tub is 7 x 7 you'll then need a 10 x 10 pit. I suppose you could build a lift out filler made of cedar or similar.
Lastly, I don't know what kind of cover you have but the easiest to use lift assist mechanisms will not work with this design unless you leave that part of the pit open to accept the cover when the tub is in use.
I did build a similar design but into my deck with removable parts to access the tubs hardware, and an open area at the back to give clearance for the cover when swung off with the lift. I also only made the depth equal to the first step into the tub, that way you feel balanced when getting in.
Al