"Polymer" (plastic) guns are cheaper to manufacture so the makers have a higher profit margin. They are lighter and easier to carry, so LE likes them. They wear well and are not affected by adverse environmental conditions, and since the majority of LE are not gun people, they like them because they believe they don't have to take care of it. On the downside, unless it's gas operated, they recoil more because of their light weight; this is a fact of physics, not a subjective opinion. They blow up on occasion, more so than steel frame guns, which is to be expected since plastic is not as strong as steel. Due to the "flex" in the frame, they tend not to be as accurate as steel frame guns, but any of them will shoot more accurately than 99.99% of the shooters, so this really isn't much of an issue. Except for the "flex" you can say almost the same things about aluminum framed guns. Aluminum is lighter than the steel but wear out quicker with excessive use.
Personally, I prefer steel framed guns.