As per the topic, where "efficiency" is defined according to getting the most out of what you pay for.
When I started, I figured that Criminals had the most efficient icebreakers in the game: you get more strength boosts out of them than the money you put in. Given that and that they're the fast Credit-generating 'runner faction, I knew they were the faction I wanted to play.
However, now that I've got some games under my belt, I find myself revising that assessment. Their style of large increment strength boosts only save you Creds when fighting high strength ICE; against smaller ICE (which are the likelier to be encountered in general and given the Criminals' speedy style of play), they cost more than what you'd normally pay for when using other icebreakers. They're not cheap either but also come with low base strength. Lastly, though it would seem that they shine in the late game where the big ICE are rezzed and active, against multiple ICE defending a server, they eat up a lot of cash.
To be fair, I have to keep in mind the strengths and weaknesses of the icebreakers of the other factions. Anarch icebreakers tend to have base strength equal to what you pay for or at least close to it. Breaking subroutines is cheap for them. However, both Mimic and Yog.0 have no way to boost strength; they rely on the Anarch's strength-reducing tricks for them to work on more powerful ICE. Shaper 'breakers aren't cheap to operate although their cost-to-base-strength ratio isn't as bad as those of Criminal icebreakers.
Please take note that I'm not saying Criminal icebreakers are bad; I think I'm just looking over the fence and thinking the grass there is greener. I run all Criminal icebreakers in my Criminal deck, using only Yog.0 to fill in for the missing decoder in the Criminals' set of tools while preferring to use my influence on other stuff. But I sometimes wonder why Criminal 'breakers aren't more like Anarch icebreakers when the former are supposed to be fast and aggressive. Fast and aggressive means using cheap, easy-to-use cards; and as long as Mimic and Yog.0 are fighting small fry, they go right through with minimal cost. Thematically, where Yog.0 is really just an organized set of stolen passwords and access codes, you'd think that's more likely to be in the hands of Criminals than Anarchs.
I am grateful that FFG paired Criminal icebreakers with their ability to make money fast thus offsetting their costs, but I can't help wonder about this.