There is definitely a skill in writing an all-purpose rejection letter that doesn't insult - by omission, by a backhanded statement, or even by false praise. ("Well, if my story was 'finely crafted' why was it rejected?" -- not actual example.) I recently received one for a themed submission that was worded in such a way that - well - I freaked out. I wondered if I understood the genre properly, where I was making a mis-step ... until I found out that someone else had received the identical rejection. Apparently, it was their generic "no" email.
This same person received an email from another location that made it unclear whether their story was being held for further consideration or bumped. In this case, I was able to clear up the confusion by observing that I'd received the same letter.
I think you have to be careful what you say when you mean to say nothing. It's definitely a difficult task!
That said, equally frustrating can be only-positive personalized rejections. "This story had great characterization and a nice twist ending." ... then why are you turning it down?
One recently made me facefault. I think I just got told my story was the last one they cut before finalizing a table of contents ... argh gibber! It was a nice compliment to receive in a way, but flail evil torture so close.