It's unconstitutional

From my point of view, this is by far the strongest argument. "Naturally we have to change the constitution to allow them," the supporters of online searches say. The problem with this is that this touches essential civil rights (Americans will know this as the Bill of Rights) that every Western democracy holds up so high. Once we start giving these up, we might just as well get out of the U.N. and stop telling the world that we're so better at treating people. I can already hear China saying "Told you so, Germany".

Lack of proportionality

Authorities claim that they can only conduct (and afford) 5 to 10 online searches a year. Is it really too much trouble getting a judge to sign a search warrant for such few incidents? Is it worth changing our constitution for this? It's not like the police isn't going to collect some evidence on the guy they're going to put under surveillance in the first place. After all, hacking into another guy's computer takes time and money, you better make sure it's worth it. So you might just as well get a judge involved.

Have they actually thought this through?

The means by which the authorities want to conduct these online searches aren't well known. The press obviously likes to speculate wildly (and usually gets it all wrong due to their lack of understanding the details).

The most obvious choice seems to be to buy a zero-day exploit for whichever operating system the person in question is using. I've heard these sell for a couple hundred thousand dollars. So the whole thing sure is pricey. Then you install a trojan (dubbed "Federal Trojan"). According to the press, this has been done already in an investigation (unconstitutionally, I might add).

The problem with the trojan is that if the police can get in that way, so can anybody else, especially if the system isn't well maintained (security updates, virus scanner, etc.). So in the end, an online search is like catching two cars on a speed camera. You know at least one of them has been speeding but you can't tell which one. That's why I doubt they'll hold up in court.

Politicians realized that too, of course, and backpedaled. Now they wouldn't use a trojan but rather fall back to more conventional methods of gaining access to the computer: breaking into the suspect's house and install the software or hardware (e.g. a keylogger) locally. Well, I sure hope that those terrorist have no laptops, otherwise them cops are goin' be real mad when they find out the terrorists always take that computer with them when they leave the house...

To me, all of this doesn't look like this has been thought through properly. Those who want it seem to think it's a silver bullit. Those who actually know a little bit about this stuff can't seem to see that it would be. I sure hope that parliament will be wise enough not to let this one slide by.