Editors often have positions in which they are the gatekeeper--the one who decides whether a piece will get published or not. But that's not the case for someone working on a dissertation. The dissertation process is not set up to include editors. The gatekeeper's role is taken by the faculty committee which has to sign off on the work.
So some dissertation writers hire an editor--indeed, some schools require hiring an editor (I think the University of Phoenix is one such, though I can't, on a cursory search, find a reference to that). Some dissertation writers hire me, others express an interest in hiring me.
I wish I'd hired an editor for my dissertation despite the cost--from the research I did when writing, I figured the cost would be two to four thousand dollars. I balked at the price then, but I would have profited from it in the long run. Although I write well enough--at least I edit well enough--it's hard to get your own documents as clean as would be best. When you're too close to a piece, it's hard to see it clearly enough to pick up the minor errors that would be easily found in someone else's work.
But the key, and what I really was thinking of when starting this, is to remember the nature of your relationship with your editor. Editors want good work. When they're a gatekeeper, that may cause rejection. But when you hire an editor to help you improve your work, the only thing they're interested in is getting your work to be accepted--that is the measure of an editor doing a good job. For professional reasons, a freelance editor wants your work accepted almost as much as you do.
Which brings me to a point I cover time and again with my clients: comments and suggestions are coming from a motivation of helping you get your work accepted. If an editor says that he or she has found a weakness, the point is not to complain or to make you feel bad; the point is to help you find a way to strengthen the work.
At the same time, the editor is not necessarily an expert: it is unlikely that the editor knows the subject as well as you, so comments from the editor ought to be viewed as suggestions to be tested against your own knowledge of the work.
It's also worth noting that editors are unlikely to be making personal critiques: it's not the point to tell the client they're wrong or that they're incompetent or whatever. The point is to help the client appear in the best light when their work is read by the people whose job it is to judge the work and decide whether it is good enough for a dissertation, or for publication.
It may help to think of the editor as a surrogate audience: if you have an editor who interprets your work in a way that you find problematic, it behooves you to address that the question, because someone other than the editor might also read the work in the same way.
Editors can be fearsome gatekeepers. But if you've hired an editor to help you finish your dissertation (or other project), that editor is there to help you. By trying to learn from the editor and by trying to understand why the editor comments as he or she did, you stand the best opportunity of profiting from you work with the editor.