You could bring down a power grid in a European nation by a well timed series of strikes on major substations and the parts to replace them just aren't held in stock (not in large numbers) and the lead time is long.
The economic damage from blacking out entire regions would be catastrophic.
The problem, from a terrorist point of view, is that if your lights go out you don't think "terror attack". A terrorist is looking for maximum publicity for minimum effort, not maximum economic damage.
It’s also hard to build bombs without killing yourself, hard to transport them, hard to deploy them for maximum effect. Then you’re likely to face the wrath of whoever you attack, and tracing bombs and bombers is something governments do well. Tracing a pathogen could be very tricky, and it might not be possible at all in some cases. Spreading a virus is something even a toddler can manage. If it becomes cheap and easy to engineer to a contagious pathogen, deploying it will be all too easy, tracing it will be challenging.
Interestingly the lack of tracability brings it's own problems (if you are a terrorist). If some group were to have claimed responsibility for the 2014 ebola epidemic, would you have believed them?
You could bring down a power grid in a European nation by a well timed series of strikes on major substations and the parts to replace them just aren't held in stock (not in large numbers) and the lead time is long.
The economic damage from blacking out entire regions would be catastrophic.