I too have an opinion. I have had many of the mentioned brakes from that era.
Weinmann; Centerpull; I have not had any undue problem with centering. It nees to be paid attention to when setting up but okay for me after that. The short coming I found with these were the brake pads. I just did not think them adequate. Replacing them with mathhauser pads when they came out solve a lot of complaints. The plastic bushing I never cared for but never had a problem with. No undue flex problems. Sidepull; The same complaint that I had with the pads of the centerpulls. the original pads were inadequate in my opinion. No undue flex problems.
Universal 68: I used to have an irrational dislike for these brakes. I have however had some on my PR-10 ,w/ chrome tange fork, for about 20 years. I recently moved them to a Sears/Puch. They functioned poorly in my opinion until I put on a better pad (mathhauser, then.) That cured them of my complaints. I did not find them to be flexie either. As a side bar that PR-10 currently has Atom drum brakes laced to 26" rims from my first Schwinn conversion mountain bike around 1981.
Mafac; Competition; This too is a great break in my opinion. They are with the earlier ball style infinitely adjustable to meet the rim angle perfectly. I have had a pair of "boosters" for at least 30 years and never used them. I felt they worked fine without them. the old competition has a better bushing than the racer. I don't care for the racer nearly as much.
Centerpulls in general can suffer from one more piece of cable to stretch. The original cables from the levers to the calipers has a minor amount of stretch. I would recommend using mountain bike cables or my favorite the original Campagnolo cables that are so nicely braided rather than just wrapped. The feel of the braided cables has a noticeably "better" feel. It is not flexy and gives a feeling of allowing more subtle use of the levers.
Campagnolo Sidepull; I found these to be great brakes out of the box with the best cables. No complaint or need to fiddle with to make just right. they were the standard of the industry and to me they still are the easiest and best.
One more opinion from; John Strizek Sacrtomato CA, USA
p.s. Sheldon Brown was so gracious to correspond with me. I too feel I have lost a dear friend. Harriet and his children have been so kind over the years to share him with us.They are owed many thanks.