There's a discussion going on in the racing sub-forum at the moment about why trainers make for a harder workout than riding on the road. Consensus is that it is a mix of overheating and the absence of the opportunities for micro-recovery that happen all the time on the road as the terrain and cadence change. Plus boredom, of course.

My view is that the lack of recovery is the big issue. Even when out on the road, if I do a two-hour workout at a steady tempo it is more challenging than two hours with the same average heartrate, but including peaks and troughs.