My agency issues 16" LMT M4s for Patrol, and 16" M4s were purchased years back for SWAT (S&W, Rock River). Folks can use individually purchased M4s as long as they fall within certain guidelines which are pretty broad. No SBRs allowed. Ironically all agency shotguns are now SBS 14" 870s, and the sub-gun is still revered. We are an all-.45 agency for duty pistols, and SWAT, fugitive, gangs, narcotics, and a few other units have UMP45s. SWAT has select fire (1, 2, unloader) and everyone else has semi-only.
I could go for an hour on the mindset behind the love of the UMP. It has some pros to it...specifically size. Our ammo (200 grain GDHP +P) does really well out of them, and performs really well with intermediate barriers and glass. It took until very recently for SWAT to be allowed to use rifles inside a house on a search warrant (that thing will shoot clear through a school, son!) and still it's met with strong disapproval from some leadership. There is a cultural change going on and it's a very slow one...often requiring retirements for change to occur.
Until then, we will have a lot of UMPs. My sole complaint about it (for unarmored bad guys under 75m) is a shitty trigger. I have a lot more time with an M4 and am generally more comfortable with it, but in some environments and for some applications (low viz/low pro, vehicle assaults) I'm very comfortable with the UMP. If I could rock a 10-12" M4, I'd shelve the UMP. Until then it still has a place for me.
