Hunting Cruise Control

I don't often use the CC but had a few motorway runs recently. I find the CC hunts, ie the speed drops slightly and then it seems to realise and speed up repeatedly which is a bit uncomfortable. Also going downhill with it set at 70mph it overruns and hits 75+. It does not hold very steady in comparison with my old 9-5. Is this normal with a 2017 S Cross or does it need something looked at.

I presume your SZT has manually operated cruise control rather than the radar controlled adaptive system.
I've not got any personal experience of using the manual system at motorway speeds so can't comment on the "hunting" issue but I have used Vauxhalls manually operated CC and that would exceed the set speed at times going down hill.
The manual CC doesn't activate the brakes so it can only "lift off" the throttle to try and reduce speed so sometimes the set speed is exceeded.
Adaptive cruise control applies the brakes as well as throttle so the set speed shouldn't be exceeded.

Yes, my car only has manual CC.  While I found the excess going downhill odd (compared to other cars) and I can control that, it is more the hunting which is 'uncomfortable'.  After a short while I turned the CC off as it was better to drive normally.

Is the CC constantly "hunting" at the set speed?
A friend at work has a 2019 1.0 Vitara SZT, he regularly uses motorways, I'll ask him whether he uses CC and has noticed any "hunting" characteristic.

Sometimes it feels OK briefly but then hunts up and down a bit. It is not much but enough to feel uncomfortable (to me anyway), almost a seasick feeling.  Although we have had the car nearly two years I have not really used it before (it is actually my wife's car and she does not use CC).  Not a massive problem if that is just the way it is as I will just not use CC.  It is an auto with 20k on it if that makes any difference.

I have a 2020 Vitara szt auto and cc stays steady at set speed.
Maybe a software update is required.

I did wonder about that but the dealer is supposed to check and update software at the service each year - at least according to the Suzuki service schedule.

I am with aussie11, I have 2016 vitara szt manual and the cruise control is fine stays steady at the speed I set. Used it almost daily for just over a year with no issues.

Probably worth getting the dealership to check the software is up to date in both the ecm and tcm.
Although there's no known concerns for the automatic boosterjet S-Crosses there was a TSB for the earlier M16A with CVT for a not dissimilar issue when using cruise control which involved updating tcm and ecm software.