I'll begin by saying I'm no expert but was thinking about this same question. A little googling led me to this page:
https://machinelearningmastery.com/teacher-forcing-for-recurrent-neural-networks/
and, in turn, this paper:
https://arxiv.org/pdf/1610.09038.pdfhttps://arxiv.org/abs/1610.09038
which as a paragraph addressing this to some degree in the introduction:
Unfortunately, this procedure[teacher[teacher forcing] can result in problems in generation as small prediction error compound in the conditioning context. This can lead to poor prediction performance as the RNN’s conditioning context (the sequence of previously generated samples) diverge from sequences seen during training
In addition, from the deeplearning.org book (http://www.deeplearningbook.org/contents/rnn.html) p378deeplearning.org book, 10, p.378:
The disadvantage of strict teacher forcing arises if the network is going to be later used in a closed-loop mode, with the network outputs (or samples from the output distribution) fed back as input. In this case, the fed-back inputs that the network sees during training could be quite dfferentdifferent from the kind of inputs that it will see at test time.
I would imagine (again, not an expert) that it is fairly problem dependent-dependent, but that the main gain of teacher forcing is in the computational training and simplifying the loss landscape (i.e. since the whole sequence will contribute to the gradient of the parameters, for a long sequences back propagationbackpropagation through time may make it difficult for the optimiser to converge even if it has a lot of computational time.)
Hope that helps!