I play in some TDM Gang War server and they have deagle drive-by and it isn't even close to being equal to combat shotgun. The amount of damage can just be modified.
I don't doubt it, but even if you just modify the damage, you just give cops another combat shotgun driveby. Then, to make it equal, you need to give terrorists M4: AK47's range is insufficient. And so on. And, again, we will end up with absolutely same classes.
Some VIPs just camp near the cop spawn (SF for example) and wait for players to drive with a cop bike and can kill him under 2 seconds, you think that is fair? The classes are all more or so equal but the damage that it can do as a passenger is just way too much.
What other weapon allows you to kill someone without reloading even once easily?
What you are talking about is behaviouristic issues, it has nothing to do with combat shotgun driveby (or any other ingame aspects). Cops can easily do the same, and they often do. As for weapons, well, you can kill anyone with full health and armour with almost any weapons without reloading. Depends on your and your prey skills.
I believe it makes it way too easy to kill the presidents (while it should be a challenge to get through) instead of just rushing with an NRG and getting it done under 10 seconds. It is never a challenge because even with pierced NRG wheels, as long as I am passenger I can shoot and wreck the president before dying. Maybe I'm the only one thinking that it's too powerful for no reason...
If it was so, presidents would never survive. But they do survive every day - not only VIPs, but randoms too. Yes, it is pretty challenging, but it is more than possible. Again, it highly depends on president's, security and terrorists' skills, and the teamwork of both teams.
Summarizing: if cops will be given anything like combat shotgun driveby, terrorists would be highly weakened. So they would be also buffed and the balance would be changed completely, likely leading to anything worse than the current situation or absolutely same classes.