Sometimes people are giving the CH the wrong impression, I agree with that.
Recently, I asked SH if I could ask a CH in a public message if they would open up other domain extensions or premium domains because I had a name I really wanted to sub. SH told me that would not be appropriate so I didn't ask. (Oh man, did I want to ask!!!) CHs may be missing out on some really great names, though, if they don't think outside of the box of a .com. There are certain categories where a .com really isn't "all that matters".
I would say: Don't ask for an increase on a 10 letter domain if it IS possible to come up with a name. Don't tell CHs that 5 letter domains are "impossible" (they are not impossible, depending on what the CH put in the brief). REAL WORD domains ARE impossible but not on all extensions.
But it is true that sometimes people say something is practically impossible when other contestants have been able to make something possible... and the CH knows that so at minimum it is just showing the CH something you may not want to show him or her.
There are times, though, when it is actually impossible. I have equal concerns for CHs who ask for something that truly is impossible and expect the results and leave mad because they didn't get that. I do think we need to "coach" them in these cases and a lot of people know when that is.