It's not that it's so restrictive.
Let me put it another way. I've got several 5100s that won't run. If they run, they won't idle, if they idle they won't take fuel well. In the last few months I've been thru 5 or 6 carbs, (some of those brand new) trying to get a couple of good ones. I've spent over 200.00 on carbs. Out of all those carbs I was able to use one. If you have a good running 5100, then hope it stays that way. If something goes sideways with the carb, a new one might not even be worth using out of the box.
I'm talking about stock saws here.....