The USP 9 all polymer 15rd magazine concerns me since it isn't steel lined.  Even Glock added steel FULL lining to their mags in the early 1990s (The earlier NFML mags had steel on three of the four sides). HK now sells a 18rd USP9 magazine that is steel lined so this calls to question is there a functional or durability issue that necessitated the change (the earlier translucent 18rd are non-lined)?  

The "all plastic" USP9 mag is the reason I have generally avoided the model.  

*Before someone says anything ... I know HK polymer is different than Glock polymer (I have owned and used both since 1994).  However, Glock is the Chevy Impala of the gun world while the USP is more akin to a H1 Hummer so I believe drawing a comparison between the cheap gun's lined mag and the superior gun's unlined magazine is warranted.*

Thus we arrive at my questions:

1) Who here has a lot of experience (e.g. duty use, high round count gun gaming, etc) with the unlined magazines?  Any problems?
2) What about leaving them loaded for extended periods of time?
3) Any concerns about leaving them in a hot car or other "extreme temperature conditions" like sub-zero winters?

Tag for info. I’ve had experience with 2 and 3. No issues there.

I have a USP40 dated from the year 2000 with original magazines and they are all three problem free. They have lived their lives loaded more often than not. I have not heard or read of any specific issues with the HK polymer magazines, but I believe they used a bit different polymer than Glock. Fiber reinforced or something like that?

The 15 round USP9 mags are steel at the top where it counts and have been in use for a long time . Rest assured that HK knows how to make a reliable mag .

Never had any issues with any HK magazines, including the polymer ones.