Well, so many opinions - here's my 10c.
Of course the voltage must be right - and the polarity at the plug - but the current? There's the catch.
Switch mode power supplies are probably better in this respect - whatever the current draw is, the output voltage is regulated to the figure shown on the case - it's built-in to the electronics.
But transformer-rectifier supplies will only give out the rated voltage at the rated current - so if you have a 12vDC supply at 500mA and you draw only 200mA from it, the output voltage is liable to be an appreciable bit higher - say 15v. To add insult to injury - the smaller the supply, the worse the regulation becomes.
So perhaps that big chunky Uniden power supply that cost quite a few dollars more might be the best bet.
For real safety - don't run your handie scanner from the charger - use that to charge the batteries then remove the charger before you turn the radio on - because as the current drops when the battery voltage rises near full charge, the power supply voltage rises some more and so on. Disconnect the charge and wait a few minutes.
If you are using a mobile style scanner on your bench, then the problem doesn't arise. Car batteries will rise to sometimes over 16v when being charged and there is also voltage spike protection in the radio - or there should be! But this is another reason for not running your handie off the car cigar lighter socket inless the adapter is regulated..... :wink: