You cannot generalize about the integrity of the mount with New FD lenses. During their production lifetime, Canon began to further distinguish between a more consumer-oriented lens lineup alongside a more professional lineup. They have of course continued this approach in the EF lens line. Plastics also began to be a viable (if not fantastic) alternative for inexpensive lenses, so Canon exploited them.
The net result is that most of the New FD mounts are perfectly adequate, while some of the less expensive consumer-targeted lenses are more failure prone. Yes, in a laboratory test, you can probably prove that an SSC-era breechlock mount is stronger, but in ordinary, nonabusive use, the new mount is fine. The only consistent exceptions are the plastic-mount consumer lenses, and there are not very many of those in the line.
I own all but four of the New FD lens types ever produced, so I can speak with firsthand experience.
Offhand, the only potentially troublesome plastic mounts I can recall are on the:
28-55 f/3.5-4.5
35-70 f/3.5-4.5
28-85 f/4
I had a 35-70 that broke when the camera strap let my A-1 hit the ground. I caught it just as it impacted. The A-1 got a concrete bruise on one bottom corner, but the plastic screw bosses in the lens either broke or stripped.
The 28-85 had a history of broken mounts, so much so that it was once difficult to find a sound example, and the supply of parts was long ago exhausted. I bought one broken, and was told by the repair shop that I got the last replacement piece they would ever get. That particular shop used to reinforce the mount somehow.
I've used FD for about 25 years, began with entirely New FD lenses, later added an assortment of breechlock lenses, and have followed FD on the internet for a good 10 years now. It's been interesting to see the rise of urban legends about the lenses and cameras, especially in the last few years as one group of users has gained a pretty extensive knowledge, another group of new users has come forth, and the direct contacts we once had with Canon insiders and even older users have waned. The legends are tending to label things in absolutes--good versus bad mounts, lenses, bodies. It was never that simple. The top-line bodies none of us could afford in the 80s are now the standard recommendations, and so the consumer bodies have begun to take a bad rap (though that trend has reversed of late on this forum). What was, a few years ago, a limited issue concerning the ruggedness of the mounts under severe professional usage--I remember a professional's report about a new-mount lens breaking during a rough whitewater rafting shoot--is now morphing into a legend that the new mounts aren't good. Soon, people will all be thinking that the new mounts are poor across the board. Too bad I already bought those . . . their value ought to plummet as their reputation suffers.
I have a shortwave receiver built in 1954. I can barely lift it. I need help to get it onto a high shelf. It will withstand a collapse of the house. It's wonderful, but it's overkill from a standpoint of need.
As one of the last bastions of knowledge about FD, I think we need to be careful not to distort or exaggerate.
-----
About coatings: the earliest FD lenses include some Spectra Coated lenses, S.C. for short. Shortly thereafter, most but not all of the breechlock lenses were upgraded to Super Spectra Coating, S.S.C. The name rings advertised this by showing the abbreviation. The long-established fact is that all New FD lenses except the 50/1.8 were Super Spectra Coated, though it was no longer such a selling point that it was engraved on the lens.
About optical designs: some lenses did change between breechlock and New FD versions. Not all. Some lenses were never offered as breechlock lenses.
About functionality: both mount types are functionally identical. The camera will never know the difference. If it says FD, it's FD.
Other differences between new and old: filter sizes, hood sizes. New FD lenses are typically smaller, lighter, focus closer, and often have smaller minimum apertures. Again, some lenses were only developed after the new mount came out.
-----
Unless you're planning to make huge enlargements or digital scans, I believe that you will be hard pressed to note any significant difference among the various FD lenses under realistic conditions. Most differences under more extreme conditions will involve image quality in the corners of the frame and/or at maximum aperture. I recently shot two identical images with my 300/4L, seconds apart, using a very stout tripod in bright sunlight. The first is so sharp it's frightening. The second is noticeably unsharp. Technique is (nearly) everything.
-----
I just held my 300/2.8L in my hand. There is no plastic to be found in that mount. The aperture ring is plastic, but that's not the mount. Like all the FD supertelephotos, it uses six screws to secure the bayonet ring, instead of the three used in the smaller lenses--and the mount only has to support the camera body in this case.
