I didn't know that. Can you elaborate please? I assumed (it seems wrongly) that threshold is threshold
Threshold is threshold, yes, in as much as you are holding the same % effort in both cases - but up an incline, your cadence will probably be slower, your position will almost certainly be different (ie you will be climbing and probably more upright, rather than crouching low on the drops), your gearing will be different and your speed will (obviously) be lower. You need to be familiar with holding the effort in the position you can expect to be in when it counts.
So in other words, the rule of specificity applies. If your plan is to ride flat circuit races, then IMO the best way of achieving threshold workouts will be on the flat, in the position which you would normally race in at speed. If your aim is to ride at threshold up hills, then riding at threshold up an incline is certainly not a bad way to develop it. Obviously if you ride hilly events, then a combination of both makes sense.
It goes back to what you said earlier - about finding it difficult to hold the same effort on the flat as you do on climbs. I know people who can ride at a given wattage up some long climbs in the alps - but they can't hold the same power on the flat for the same duration. Anecdotally, I don't have an detailed explanation for that (other than higher speed, more resistance, etc), but it doesn't seem to be unusual.
