Here's an example of a plot device, but first I'll set the scene:
Following a numerous compels on one PC's HC (Special Investigations Lead Detective) and Trouble (Unbelieving Bureaucracy) over several chapters and another PC's habit of mouthing off to the police, there's a major reaction from the police. There practically has to be - things (including of the long running themes) have been building and building and if nothing happens now then there's a huge plausibility gap if nothing huge happens. The group is currently split, taking care of different things, and the GM has the PC who's a police officer make a contacts roll.
The net result is the PC who's a police officer calls another PC and says: "They're pinning this whole mess on you. You can expect the tactical unit, SWAT, the FBI, Homeland Security, and maybe a hundred local cops to come to your door as soon as they get their warrants. You've got maybe an hour.".
The raid - it's a plot device. There's no way to stop it. There's no effective way to fight it. A small army of heavily armed, nervous people are going to show up and if the PCs try to fight then it's the battle of the Alamo or Butch Cassidy's last stand. Even if you can take out fifty or sixty goons the others will fill you full of led and they will be shooting to kill - especially if "officer down" has been called. With that many people involved, any PC who's there when the raid happens is going to go quietly or in a blaze of glory.
So the question is, how do the PCs react to the raid? Do they run, go underground, hide out, veil and watch the operation, use this as a distraction to hit another target, have sniper positioned to take out officer Rat Face (long standing NPC who's behind the raid), or do they do something else?
And a complication - if they run and leave the apartment like it is, the lab boys will find all sorts of evidence of weird stuff, enough to keep the investigation going for weeks (or months) but if there's no evidence the investigation will eventual peter out - so there would be no need for everyone to get fake IDs and move to another city.
How many cops are involved in the raid and how preciously are they armed? That's irrelevant - they have enough to gack anyone who resists arrest. That's why the raid is a plot as opposed to a balanced encounter.
Richard