It also depends on where you go and which company is handling the excursions. Recently, in Mazatlan, I found that all the taxis were new. But none had seatbelts in the back! Apparently, almost all taxi drivers had covered their backseat with a very nice and clean seat cover, but when installing they either removed or covered the seat belts. I searched and searched and the cab driver searched and searched, but we couldn't find any complete seat belts. Yuck. I drove my husband crazy by moving from cab to cab.
About busses, most of the excursions we have gone on have been with large busses (not van style), but there are always exceptions. Transportation for excursions is the only negative for me when cruising -- we love the rest of it!