What I find with long journeys, without regular breaks, is my lower thigh seems to develop mild cramp from inactivity
My old Lexus IS200 had full electric seats and it was very easy while driving to raise and lower the front seat squab to put pressure in different places that relieved and removed the cramp
Not sure how much is due to the seats, and how much is inactive legs when using cruise control. Even without ACC your right foot can be relatively inactive on the accelerator pedal and get cramps etc. On long trips in light traffic ACC allows me to do some gentle leg and foot exercises. Nothing so drastic it takes my feet, or attention ,so far from the pedals that I cannot immediately take over control if required.
On a similar vein (rather than deep vein thrombosis
) I recently read an article that claimed basic ,dumb, cruise control can increase accidents by 8%, while active cruise control 'only ' increases them by 1.8 % (IIRC) . But Lane keeping assist (LKAS ) reduces them by 19.1% (It didnt mention RDMS)
I think it may have dumbed the statistics down a bit ,and possibly sensationalised by a journalist looking for a good story. It probably only applies to those incidents that can be directly related to such inattention on motorways.
LKAS can save you from inattentive lane wandering. But cruise control can tempt some drivers to be less attentive than they should be. If you remain properly vigilant ,with ACC only as an aid , it can ,IMO still be a positive thing for road safety.
