If I say "That isn't~" instead of "Which isn't~" here, would it make any difference in nuance?
Native speaker’s answer
Rebecca
It would sound a little awkward in this case to say "that is" instead of "which is". This is because "that" is used for defining clauses when you are presenting information critical to the meaning of the sentence. In this case, "isn't always great for passengers" is an outcome of selling out tickets on a plane, but it is not essential to the original meaning of a sentence. Therefore, it is more common to say "which" instead of "that" here. Here's an example of "which" and "that": Ex: My cat that has black stripes is outside. Ex: My cat which is black is sitting on the couch. The first example sentence implies that there is more than one cat since "that" is used. However in the second sentence, "which" tells us that there is only one cat. The fact that it is black is not essential to the meaning of the sentence and so it is a non-defining clause.
