Our tub is recessed in the deck, but not flush. I'd have to measure but it's roughly 15" or so give or take a few.
We barely got to use the tub after buying the house, because of all the issues with it, however, getting in and out was very easy.
I could imagine a flush tub would be much more difficult, like stepping out of a 40" hole in the ground. With ours, I just stand up, step on the seat, and step out and over. The seat is only slightly lower than the deck, so you aren't really stepping UP at all, if that makes sense. You're more so stepping OVER the tub edge; it's really a piece of cake.
Because our deck is so large (~40x40) and multi-level, in order for the tub to be where it is & for the deck to be only 1 step down from the house, there's actually a smaller sub-deck built under the tub. A similar issue to what the OP described, they would make an entire lower deck to match up, the previous owner of our house didn't want that, so they built a deck to hold the tub where they wanted it. It's pretty beefy too, considering the tub is 800 gallons.
So there's essentially a 93x130 deck built under my deck, so the hot tub can be where it is; the proper height, and just outside 1 of the back doors.
