This has been my experience too. I've been on a few! I quite enjoy them... You often get to visit somewhere new, and even if it isn't, there are usually special exhibits or talks that are done for school groups that aren't offered to the general public.
I have yet to be dumped with the PP's experience of being sent off to fill several hours of unstructured time. It has always been a tight schedule, often has a planned craft in the middle, and I have been accompanied by a member of staff (with their own group of 4-6 DCs to keep track of) as well as often a member of staff from the museum/similar too. Some parts are all adults and DCs together, which can make it harder to keep an eye on your assigned group.
I take travel sickness pills myself plus a few boiled sweets to secretly stuff in my mouth if needed, as I get motion sick. I have never needed to deal with a sick DC - this is always the domain of the paid staff!
I've always been purposely given an easy bunch of DCs (engaged DCs, no behavioural issues, no medical needs), although that doesn't stop the anxiety of potentially losing someone else's DC! You will be constantly counting them! The teacher will regularly count the whole group throughout the day too, particularly before and after any transition points, so there is very little chance of losing one in reality.
You should be asked to read and sign a risk assessment, so you will know the who/where/whys from that.