When is it bad to give up our english or the dragon bishop againts its analog

1. Sometimes it seems like that exchange should be avoided.
2. Sometimes people say it costs too many tempi for the player who forces that exchange and can result in an advantage.
3. Sometimes I read in a book that it's not a good plan because it lets time to take action in the centre.
4. In some opening book (about the Pirc/Modern) that exchange plan was even called the "1800 attack" meaning that it's a bad plan that's chosen by weak players (actually I am lower than 1800, so maybe I should care!)
5. I read that in some situations it was advantageous to even sacrifice the rook in order to keep that bishop, by moving the bishop to the very corner by playing ♗h1 or ♝h8.
6. Besides, I know there are a plenty of singular situations such as in the Dragon, when the plan is just a plain mistake, because some tacting allows Black winning a piece are a pawn. But I would like my question to be considered for all situations other than those exceptions.

Thanks for any help!