My Rivs, at least, complain if I put much more than 15 lb more or less evenly distributed between L and R; though 20 -- 10 each -- is alright. Heavy loads do much better in the rear, either in Rollers or in one of the big Riv saddlebags -- I've carried up to 45 lb in rear panniers on a light road frame (v good rack), 40 on the various Rivs; usual rear load maxes out at ~35, though. But much better than front.
Dave Porter added front lowrider bosses to my original issue Sam, and I tried putting 40 lb (perhaps it was 50; at any rate, a very large number of gallon milk jugs filled with water, to test things) in lowrider panniers. I could barely steer, and turned around within 1/4 mile of home.
Might as well complete the anecdote train: Long ago, had a cheap Schwinn World Traveler or somesuch. Installed huge Wald Newsboy basket and slung in a couple of cinderblocks -- test; don't generally carry contruction materials on my bikes -- for a total of (IIRC) 53 lb. Talk about unwieldy! I did ride it for a hundred yards or so.