From the research I've come across, much of the problem is from the brass itself rather than the chamber (though it likely has much to do with it).
If you consider the size of the .22mag case, and how much resizing is done to create the .17HMR (or hm2 for that matter), the % in change of size is pretty large. Re-working the brass this much can cause a host of problems, one of which is becoming somewhat brittle.
If you look at unfired cartridges you can see the striations in the neck area do to the stresses the case has been put through during it's initial sizing. A little heat and pressure can cause one of the weak striations to split rather easily.
The manufacturers of the ammo have been contacted on this subject many times, as well as different firearm manufacturers...the general consensus is that it isn't that big of a deal. It isn't affecting velocity, or causing gas leaks back through the bolt. It isn't resulting in jams, chamber erosion, or anything else (or so they say).
My marlin 917v has 2-3 split necked cases per box of 50...my group size doesn't see any difference.
While i still keep an eye on the spent brass, I'm discounting it as not much of a problem....it seems quite common across all platforms--and has been discussed to death at rimfire central.
That being said, firing out of battery is a huge issue. It seems to be very common with the Remington 597 series of rifles chambered in the .17's.
The consensus is the small bottle neck cartridge requires a more strict chamber cleaning regimine than your normal .22lr/.22mag. A small amount of powder residue (with I know my HMR leaves plenty) in the chamber can cause the round to keep from fully seating.
This is why many semi-auto rifles have slightly looser chambers--for ease of extraction, and better reliability. Unfortunately, this usually turns into a deficiency in accuracy as well--something those .17 shooters don't want to see.
Last would be issues with conversion kits. The .17hm2 conversions require extra weight in the bolt, or a stiffer recoil spring to keep the cycling process (blowback design) from starting too early. If the correct stiffness is not acheived, it is possible for the case to be extracted before the pressure has relieved enough--and you end up with powder and gas in the face.
I guess what I'm saying is...I wouldn't be caught dead with a semi-auto .17...as there are too many issues with them, not necessarily due to manufacturing flaws, but when you consider their miniature size and shape and of cartridge...too many things can cause too big of problems in a blow-back designed semi-auto.
