We have never taken 2.5 yr old DS on a coach without also taking a car seat for him. We deliberately bought a second seat that is suitable for use on coaches for when we occasionally use them, here and abroad - airport transfers etc.
We have taken him on a train though and it's always been quiet enough for him to have a seat to himself or it's been a short trip in the buggy. Same for buses, which we use v occasionally.
My in laws asked to take him on a day trip 1.5 hrs away, reached by 70mph A roads. I agreed but then learned it's a coach trip and he wouldn't be able to take his car seat as he wouldn't have his own seat for the journey. I then said he couldn't go by coach but they could self-drive him in their car if they want to go. They said that wasn't an option so have cancelled their plans for the trip. FIL then started asking why we take DS on trains, buses etc without his car seat and what's the difference etc? He's clearly miffed with our decision (DH agrees with me). To me there's a clear differences in terms of safety and besides, DS is unlikely to sit on an adults lap happily for 1.5 hrs one way, even if they took him without his seat.
Probably one for AIBU too but interested in less fiery thoughts too, so posting here for views please. I feel strongly about car seat safety - DS' main car seat is rear facing - and can't be comfortable with what the ILs want to do. Fair enough?