Use chat when its facilities are more suitable for improving the quality of the Q & A.
Comments are (intentionally) very limited, and quickly become difficult to follow and work with as the number and complexity of interactions rises.
The lasting value of our site lies in the Questions and Answers, not comments or chat.
(For general background, see the chat FAQ and How to start a chat?)
It is rare to need to create a chat room manually to address the problem of long comment chains:
Where a "comment conversation" seems to be developing, the system will automatically prompt those involved to continue the discussion in an automatically-created chat room. (If prompted, you should normally take advantage of that offer).
Many once-legitimate comments become obsolete over time, as their intended purpose is realized (most often by an update to the post they are attached to). Comments such as these can be flagged for deletion at that time.
An excessive number of (valid) comments on a question is a fairly strong indication that it is not clear or specific enough to be answered, or perhaps simply just not a good fit for the Q & A format at all.
These types of questions likely should be placed "on hold" until they are improved (perhaps using chat). Once the question is reopened to answers, obsolete comments may be flagged for removal as normal. Unsalvageable questions will be closed and usually eventually deleted.
Excessive comments on answers likewise often indicate issues that might not be resolvable (in comments or chat). Poor quality answers tend to attract downvotes, reducing their visibility. Unless improved, they may be deleted by the original author, trusted users, or a moderator. Incorrect answers are not normally deleted, since they add value by usefully highlighting what not to do.
Moderators may choose to create a chat room in response to system- or user-generated flags, or situations they encounter directly. 10k users have access to additional reports that identify highly-commented posts.
Anyone that encounters a post with an active, on-topic discussion that would be better hosted in chat may raise a custom flag (with their reasoning).
Comments have a pretty narrowly-defined set of valid uses, and are a poor venue for discussion, due to their (intentionally) very limited feature set. Chat provides a much more flexible arena for real-time discussion (including improved formatting, image hosting, and better notifications).
It is pretty tough to think of a scenario where it would be useful to create a Q & A chat room ahead of time (i.e. before any comments have been posted). The sort of question that would need a chat room to become answerable is pretty unlikely to be on-topic and suitable for our format in the first place.
It is difficult to anticipate every situation you might encounter in advance. The moderation team and other experienced users are available to assist with specific situations in the main site chat room (appropriately enough), and here on meta.