As it is now, it seems a lot easier to defend on most maps than attack.
I think disarming C4 should take longer than arming. Maybe not twice as long, but it shouldn’t be as fast. It could be explained by “encryption” or trying to figure out the wiring.
I also think any objective that requires repair or construction should not be either once destroyed by C4. The objective might be something that a few twists of a set of pliers can repair, but re-assembling something after it has been blown to pieces once in the same amount of time seems a bit of a stretch.