The main issue i have with regions is it forces all viewers into the same visualization of the code. Weather or not it is a good organization or not it forces that View of the code on the reader. When a class with regions are poorly organized it harms all viewers.
And poorly organized is relative to the ideal conceptualization you need, ie: the mental picture.
The most common and in my mind most annoying #regions are grouped by:
Why? This is arbitrary organization based on the parts and not a conceptual model. It would be like organizing a book based on verbs, nouns etc instead of the normal conceptual model, units of time (ie chapters).
Good organization would be regions based on Interface implementations, Class overrides etc.
Control visual element hook-up
This organizes based on logical separation of concepts.
However this is still not perfect. Because any newcomer to the code will be forced to view the code with the organization I chose in my regions. If that organization doesn't work for what they are doing(the conceptual model they need), they are forced to work against that model. They will end up expanding the regions, trying to block them out mentally, while attempting to make the code coherent in their mind.