I can only speak about the electricial requirements. The equipment HAS to be accessable. That leaves out a pre-fab hot tub. If you want it in ground it will have to be one build into the ground just like a swimming pool where the equipment is in a separate location.
Not at all true. When you set a self-contained spa into the ground, you simply set it into a vault. That allows drainage, airflow, and you can allow an extension of the bottom of the hole so that a tech can hop down to the floor level of the vault and do repair. How do you close off that hole? There are many ways. MANY ways. I have built a nice hatch (think boat hatch) out of some fancy hardwood like Jarrah, Mahogany, Apeton, or similar. Oil it once a year, or let it go grey, your call. Leave air spaces between the planks as you would on a boat, to allow air flow. Also this allows you to step out onto wood, which is usually not too hot in summer or ice cold in winter.
I have poured concrete lids for the hatch - takes two strong people to lift out, but you don't go down there that often. Perfect match to the concrete deck around the tub.
I have made the hatch out of exterior/marine plywood and finished it with epoxy deck finish - dead match to the concrete around the tub, including texture if you take care in applying it. May need a bolt or two on each side to keep it from warping over time. You can build it strong with structure beneath to avoid warping, but it adds weight of course.
You can put the sub-panel in the access area, I have seen sprinkler control systems end up down there too, but in any case, be SURE you allow good drainage. You will not usually get enough rain or melted snow down there to fill it up, its usually the owner who puts the garden hose in to fill/refill the tub and forgets it...
Here's one from Bullfog's site:
Here's one from HotSpring's site, with a wood deck surround. The HotSpring site offers several pages of images, along with ideas on how to set a tub down.