Why do nuke missiles & a bombs have such little effect

I prefer to send a bombs via a transport but I wonder if it reduces the impact
seems it would be easy to shoot down a transport;
if U send nuke missiles; they're incredibly slow

Well, balance.

Nukes are strong but yea slow. I don't know if those need some ''balance''.

Nope, they doesn't reduce the impact.

You are probably reffering to the types of nukes, there are some which are long and "Air"Type of units, they can be nuked from one city to another

There are some called "Broken Missile" which are "Ground" type of unit, but they have very few range. You can decide whenever transport them or not.