They're using silencers on the firing range? What's wrong with regular ear protectors?
They're less convenient. With a silencer, you only need one on the firearm that you're shooting, and everyone else in the room doesn't need to bother with protection (assuming subsonic ammo). Shooting rifles with ear muffs can also be awkward when trying to get a good cheek weld.
But still, both of the main types of suppressors negatively affect both control of the firearm and impact on the target, and some accuracy.
What makes you believe that?
I won't claim to be an expert marksman and have a nuanced opinion on accuracy, but I do know from personal experience (owning both guns and silencers) that any such effect, if it exists, would only matter for precision shooting (where you want sub-MOA groups, say), and is completely irrelevant when shooting at people.
Control of the firearm is affected in a sense that it becomes larger and heavier, and specifically more front-heavy. It's a bad thing in a sense that it makes it harder to draw fast (mainly a concern for sidearms), and it makes it bulkier and harder to carry around in your arms (not really a concern for police, who generally shouldn't be wielding their weapon for prolonged duration - they are not soldiers). It is a good thing in that increased mass means lower perceived recoil, which means more controllability for repeat shots, esp. in rapid fire. Furthermore, suppressors themselves reduce felt recoil by virtue of their design, further increasing controllability.
In terms of less stopping power, it depends on the ammo. Obviously, if you want your silencer to be effective, you're going to be shooting subsonic ammo, which means significantly reduced velocities. But this is generally compensated by increasing the mass of the bullet, or by using a caliber that is subsonic to begin with (like .45 ACP). Then there are calibers that are specifically designed for suppressed subsonic use, like .300 BLK, or 9x39mm of Vintorez fame.
So for sidearms, for example, if they are chambered in .45 ACP (which is generally considered as one of the better handgun calibers in terms of stopping power), the stopping power will be exactly the same with or without a suppressor. Ditto for .45 SMGs.
And, frankly, the only reason why I can see police ever needing supersonic bullets is if they have to confront someone wearing a bulletproof vest - which is a pretty damn rare occasion in practice.
Let's not pretend that the DoD equipment being shipped to local police forces has anything to do with improving policing or protecting people's safety.
This (as well as "we have plenty of evidence that police using firearms in public negatively affects the health of people at the scene") is a different issue. DoD ships a lot of equipment to local police forces, and most of it doesn't really serve a good purpose, and a good chunk of it does in fact induce abuse - but silencers would probably be somewhere on the bottom of my personal "offensive" list. Fully automatic firearms, armored cars, .50 BMG rifles and other such things would be on top. But really, the entire program itself is messed up in too many ways, especially when combined with "equitable sharing" asset forfeiture. I would be perfectly content to shut down the whole thing entirely, and have the local PDs pay for any equipment that they require out of their budget.
As far as silencers go, I would actually be willing to go so far as to mandate their use by everyone (including police officers, except in special circumstances) when shooting in public for any reason outside of designated ranges - including during hunting etc. Given the permanent hearing damage that can be induced in bystanders, I think it's irresponsible to not take care of the noise when there's the ability to do so. Of course, this is not viable so long as silencers remain regulated and taxed as heavily as they currently are in US (much more so than guns themselves).