They are made for penetrating only light clothing.
They don't ricochet, a glass window or sheetrock pretty much stops them.
If they penetrate in the center of mass, you're dead.
If they hit your arm, all that's left is the bone.
They have a blue polymer tip followed by 1200 BBs in a jell.
Due to the rotation of the bullet, the BBs come out in a swirl and make about a three inch cavity.
A normal leather coat or heavy jacket will usually stop a BLUE Glaser.
In the winter, your use GRAY tipped Glasers. They will penetrate heavy clothing.
If I was a cop in a city, I'd be using Glasers to reduce the chance of harming an innocent.
The issue with over-penetration is that it is only an issue if someone hits their target, and the bullet makes it through and goes on to hit an innocent. For most modern HP designs, that should not be an issue as the most modern HPs do not clog the hollowpoint with clothing (which would turn them into, basically, FMJs). Modern hollow points like the Federal HST, the Winchester Ranger Bonded, and the likes of Critical Duty, do not clog (especially when compared to older hollow points like Federal HydraShok and Winchester Silver Tip, which often clogged).
The big issue is this - missing! If a cop misses his target, then even the most advanced HP design can hurt an innocent. Even Glasers (note though that most cops would not use Glasers as most cops require penetration, and glasers don't have that. This is why many PDs use bonded bullets, or non-bonded bullets that hold together well).
Why is this an important point? Because most police officers are very bad shots (missing most shots taken according to police data). Which means that no matter what type of round they are using, it doesn't mean much if they miss their targets and hit innocents.
To make matters worse, NYPD guns have very hard triggers (12 pound pull, every pull) which makes accurate shots - especially if they are blasting away quickly - very difficult even for people who are normally good shots. Put such hard triggers in the hands of people who generally don't practice that much on average, and the situation is significantly worse.
Anyway, the round used by the NYPD is the Speer Gold Dot HP, in 124 grain (and a +P). It is a bonded bullet, which means it holds together very well (even after hitting auto glass, which normally messes up non-bonded bullets). The Gold Dot has a long history of very good performance, and while it is older than more recent bullets like Federal's HST, it is still a very good round that will do its job if you do yours.
I find it very hard to believe that a bonded bullet was unable to penetrate a jacket, and I suspect that there is something more to this story that is not being reported. Either there was some untruthfulness, or maybe the bullets hit something in the jacket, I do not know. But it is worth noting that the Speer Gold Dot is a bonded bullet with many years of superlative performance.
As for those who are saying the issue was the cops were issued with 9mm and not 'manly' .45s, the truth is that the 9mm is easier to control, has higher carry capacity, and (with modern HPs) is just as lethal as .40/.45. There are enough studies backing this, ranging from the recent FBI study showing the 9mm as the most effective defense round with modern ammo, to reports from police on their inability to differentiate between wounds caused by 9mm HPs and .45 HPs. Indistinguishable.