I compared the early ranged units. I'll abbreviate melee strength with MS and ranged strength with RS.
Unit MS RS Composite bowman 7 11 Catapult 7 8 Crossbowman 13 18 Trebuchet 12 14 Cannon 14 20
You get composite bowman and catapult about the samt time in the classical era, and the crossbowman and trebuchet is available in the medieval era. Also roughly about the same time.
The composite bowman is (as far as I can see) better in all aspects compared to the catapult. Same goes for crossbowman and trebuchet. And the catapult and the trebuchet have the huge drawback that you have to set them up before firing, which means that the enemy gets the first shot. The cannon which comes in the renaissance era is slightly better than the crossbowman, but not by much.
I know that the siege units (catapult, trebuchet, cannon) gets a bonus when attacking cities, but apart from that, is there any reason to use them? The only reason I can think of is that when you upgrade a crossbowman you get a unit with less range, which is a huge drawback, while the cannon upgrades to artillery that has 3 in range.