vs the steel spring is the increased cocking effort relative to the output. Another "downside" (to me again) is the rather sharp firing behavior compared to the spring.
As you can tell, I'm not a fan of gas rams and much prefer a well tuned springer. A few years ago I shot a Crow Magnum and that beast was a real gut buster to cock, however I've shot a steel spring Kodiac and had no trouble cocking that particular beast, and that bone stock .25 cal Kodiac was putting out Diana domes at an even 800fps (almost 30 fpe). Regardless of the sales pitch, rams CAN leak down and require "expensive professional help" but springs in most guns can be cheaply replaced by a mechanically minded owner when needed every couple years of heavy shootin' (say a box of CPs a month) if the gun has a proper aftermarket Maccari spring.