Originally Posted by bulgie

Front drive was preferred by classic constructeurs like Herse and Singer. Main advantages are increased chain life (chain bushes only wear when touching a sprocket, which they do less than half as often) and no chainline issues when cross-chaining. Chainline becomes a non-issue since it's the angle that the chain cares about, not the actual lateral displacement. Singer in particular used that fact to put 4 rings up front for a super wide range, which is possible but impractical when the crank is in the rear (or on a single bike). On a front-driver, there's no downside to a quad. Also with front drive you don't need the indent in the right side chainstay for chainring clearance. Oh and one minor advantage has been mentioned, that cap'n can see and hear the front shifting and any rubbing.

Downsides are mostly the extra weight of that much chain, and sometimes the rear derailer needs help keeping that chain tight enough to not sag too much or flip around. Not all rear mechs can handle it well. The old Cyclo that attached to a chainstay braze-on was perfect for that because it's spring was like a screendoor spring stretching forward under the chainstay, and you could easily just swap in a stronger spring of the same length.

Affect on Q-factor (aka tread): Stoker's crank needs to be wide on the right, so the chain doesn't hit the right crank. Basically a triple spindle even though there's no chainring on that crank The cap'n cranks can be narrower, since the limit to Q in the rear is tire/chainring clearance for the chainstays and we have removed those constraints, plus chainline doesn't matter anymore, so whatever the narrowest spindle is for your cranks will probably work fine.

Modern builders have almost 100% dropped the idea. Not sure which downside most affected that collective decision; probably fashion had a large part in that. Of the "real" (non-fashion) reasons I'd say the main one is the need to use certain derailers that have adjustable spring tension. Less of a problem for a C&V fan since there were several to choose from BITD. Now, I don't know, I don't play with modern derailers at all but I assume they are not adjustable, is that right?

Mark B