You need to design the system within the limits of the chain. For us, with a few of our drivetrain gearboxes, we were exceeding the maximum reccomended working load for the chain, but were still under the chains breaking load - just barely. We have tested it thouroughly enough to know that there is an obvious safety factor in the chain manufacturers quoted numbers, and as a result have designed our systems carefully enough around that, knowing one small misalignment or potential impact during a match could mean we break a chain. We take that risk, at the benefit of reduced weight and size. It is a fine line to walk, and you have to consider everything before you make that decision yourself.
Transfering 'force' should be clarified, too. #25 chain would probably transfer some force in this picture below just fine, however, the massive reduction in the gearbox means that as soon as you put any sort of 'real world load on the system, the chain will simply fail. The amount of force you wish to transfer has everything to do with what size chain you need. Below, we almost needed a chain larger than #35, but again, we figured if there was some sort of impact or collision during a match, we might actually prefer the chain break rather than the arm or frame of the robot itself.
For drivetrains, it's worth running the numbers every year just to be sure that #25 will work. You can calculate this quite easily. 2 cims at 2ft/s would be safer with #35 chain in my book.
