While a journal editor is unlikely to completely reject your paper for missing a deadline, there are things you should consider. The first is that you should keep the editor informed if you will miss a deadline, ask for an extension, and provide a realistic estimate for completion.
One reason for providing a deadline is just to keep you working. But note that an editor will typically have several issues in preparation at the same time. There may be restrictions on total length of an issue, both minimum and maximum, and the editor may already have a slot in mind for your paper, provided that you complete in time to fit the production schedule.
If you miss the deadline, then, while the paper may still be accepted, it may be delayed in publication, possibly for quite a while. That might be especially true for a long paper.
Of course, if there is no physical version of the publication and everything is done online, then the length is less of an issue. However, the editor may also have an idea about a theme for each issue and your paper may fit that theme for an upcoming issue. If you miss deadlines then it may take even longer, until that theme comes up again or a more general issue is created.
It is to your advantage, in just about every way, to work with the editor to make his/her life easier. It is a mutual thing. In particular, don't make assumptions that, if incorrect, make life difficult for both you and the editor. It is easy enough in most cases to find out what you need to know.