Well let's look at the IW UUs:
Prats - essentially a cheap early mace. These suckers cost more and the tradeoff between saving

vs spending

is particularly bad with them. The whole reason they rock is because they can take down everything until LBs easily; one of the UUs with the longest lifespans makes a lopsided

to

trade particularly bad.
Gallics - they can be built by Cu and come with GI. These guys have three main strengths: they can double move across with the use of one promotion, with a good setup that can mean smacking an AI cap on turn 0. They can defend and attack well with GII, again though this is highly terrain dependant. With GIII (must win a few fights) they can get withdrawal odds. Lastly they are strong units to upgrade into (C1)CRIII/GI(II/III) rifles. What does getting there early with fewer of them mean? Well you might be able to poach a few cities if the AI doesn't have Cu that would otherwise not be viable with a larger stack. Still, this doesn't seem all that impressive outside of a map like highlands/lucky tectonics. Hills are rarely that closely packed and if you can build gallics by chopping out the oracle because they can be built with bronze then you could have chopped BHPOBA (Big huge pile of bloody axes); marched them into the AI cap, and have been done with it. While axes are slightly less efficient vs archers on hills; you can get them earlier before the AI stops building settlers/workers, they are stronger vs axes, and you can use the oracle

to get 11 bonus axes. With Brennus, I'd be far more tempted to just go for HAs anyways (worker techs) -> writing -> HBR -> archery is excellent trade bait, quick, and good for CHA leaders.
Jags - cheap, low strength, come with WI. Jags have three main uses: they make the best uberMASH in the game (CI/WIII/MIII - add morale for fun) with the sole possible exception of the Impi; this is irrelevant to when you get IW. They can double move with WII and poach cities if the AI hasn't clearcut yet. They can be hoarded with no prep and require no settler to find the resource.
Jags are best in numbers, before the AI converts its trees into axes. This does make them a decent candidate for an oracle shot but ... seriously this is MONTY. We have the sacrificial alter, a religion, and massive trade bait just waiting in CoL. Getting up alters can easily let you get 50% more units and make your recovery massively easier.
I really have a hard time seing where swapping all those

for early IW actually pans out.