Last time I had the misfortune to be on a rail-replacement bus was about three years ago now, between Rugby and Northampton. My train arrived a few minutes late from BHM to find that the bus had departed on schedule, without being held. The driver of the following bus an hour later couldn't find Long Buckby, where he was supposed to stop, kept stopping to look at a map, turned round on a petrol station forecourt and again in a farm driveway near a blind bend... Eventually found Long Buckby village and set down one passenger but didn't try finding the station itself to pick anyone up..... Yes, I did mention all this to train staff when we eventually arrived at Northampton!
The same year (don't remember which came first, now) a late evening Chiltern service from Marylebone to Birmingham Snow Hill was terminated unexpectedly at Dorridge due to cable vandalism - something we on board hadn't been warned about until ater leaving Leamington Spa. A replacement bus had supposedly been lined up but arriving about ten mins late at Dorridge the station was deserted and according to a note stuck in the closed ticket office window the bus would have been scheduled to depart a few minutes earlier and clearly hadn't waited. If it hadn't been for the departing Chiltern driver being apprehended by us eight bemused passengers on his way into a taxi waiting to take him to Tyseley and being asked to contact the company's control point (no, the on-platform 'help' buttons were merely giving out-of-date recorded information and it was way too late at night to get hold of anyone on any other advertised contact number) we'd have been left to our own devices. It still took nearly an hour and a couple more phone calls on the number he left with us before two taxis turned up.
So, funnily enough, I've always been very wary of replacement bus services since then........