1. I don't know how much your Filco was used but mine after heavy use (all keys shiny slick, shine almost everywhere between keycaps and under space bar, it was never opened by anyone though) was (yes was - I swapped it for another Model M -hehe) still better put together than any IBM I ever had.
The Filco I owned was new, and I seriously doubt any Filco could stand up to the intentional abuse I've seen some Model M's take(eg. in YouTube videos from individuals attempting to destroy them).
Model Ms also creak, less than Unicomps but still...
Unicomp cases have more flex, and when flexed they tend to creak. It doesn't take much effort at all, which is why they're so prone to creaking when simply pressing down on the case with a moderate amount of pressure. IBM, on the other hand, used a much sturdier plastic, and as a result Model M's are relatively inflexible comparatively speaking, and hence are relatively creak free. You can make them creak, but it takes far more effort to do so.
I think it could be something to do with plastic composition (IBM's ABS vs Unicomps PBT/ABS mix) but it's a fact that Unicomps creak more - I don't care about that though.
Perhaps you don't care, but the issue is indicative of the difference in build quality and is extraneous to whether the Model M is used or new.
but about the abuse they went through and dirt/corrosion they accumulated.
It depends. I've purchased a number of used Model M's on eBay in various stages of disarray and they've typically cleaned up nicely and functioned as expected. Obviously there are non-working units being offered on the marketplace, however I've yet to run into an M that was so dirty and/or corroded that it made a really noticeable impact on aesthetics or performance.
I always look for IBM manufactured Model M's(80's & early 90's), sometimes intentionally looking for M's that appear a bit dirty specifically because they generally work, can often be had for a lower price and are easily cleanable(inside and out), often to the point of looking practically new. They're relatively easy to refurbish, assuming you don't need to replace springs or worse. However, even that's not really an issue considering Unicomp will replace broken springs on vintage boards if you send it in(for a nominal fee of course). Me, I honestly prefer my refurbished and / or used vintage M's to my brand new Unicomp. That's me, of course.
If I had choice between brand new IBM with 104 keys and USB and Unicomp I'd probably choose IBM due to a bit higher build quality, but when I'd have to choose between 20 years old IBM with 101 keys and PS/2 + flaky adapter (yeah all of them are flaky, blue cubes and belkins included)
In my experience, not all of them are Flaky(none of mine are). I own a Belkin ADB, Bluecube and Griffin iMate. The only keyboard that's ever acted flaky is my Chicony KB-5181, but I believe the keyboard is at fault rather than the adapter because every other keyboard I own works fine in conjunction with them.
and brand new Unicomp with 104 keys and native USB, with warranty and superb support it's no brainer for me, especially that those IBMs often cost more than Unicomps, especially when they're NIB or restored condition.
On the whole, all of the used IBM manufactured Model M's I own are superior in build quality to my Unicomp Classic and some cost less to boot. Earlier M's and F's vary in build quality in respect to one another, however that said, I'd still put my Unicomp at the bottom of the list, though. That doesn't mean I don't like it. I simply prefer it less than my Model M's. Again, that's me. Obviously you feel otherwise.
