Actually all my Zenits looked great, even smelled good.
The reality is they just suck compared to what was being manufactured in Japan and elsewhere. If they didn't they would have been used by professional photographers all over the world. But for some reason they instead used Nikon, Canon, Pentax etc. If Zenits were any good those would have been used.
define "suck"...
Most known Zenit ie, the 3M derivatives in M42 mount: E(x)/1(x)/V are basic cameras with limited shutter times because they are just build around the Leica II style shutter, no complex gearwork nor clockwork, just geometric distance intervals between notches. It's very basic but the simplicity goes with reliability.
Lack of slow speeds 1s to 1/15 and lack of 1/1000s brings limitations of course. So it's about features.
To use again the cars analogy a VW Beetle isn't a Porsche 356 nor 911, but is it a bad car?
these Zenit weren't meant to be for professionals and in fact soviet industry knew very well they had a dilemma between the need for professional gear and the philosophy of economical affordability and production costs control, which did limit in the first place for ideological reasons manufacture of high-precision heavy-duty cameras.
So they kept manufacturing mostly (not only) simple low-tech but reliable systems and easy to maintain.
My S, 3, 3M, V run perfectly. But then if you tinker a bit, you can pick any old Zenit with slow curtains, jammed shutter, uneven times, and fix it yourself, because it's so simple. The most time consuming is replacing or reglueing curtains ribbons. Then you have a simple yet good and reliable shooter.
Lack of features isn't bad per se, bad is bad design or design/implementation mismatch.
An old Zenit isn't good for handheld shooting at slow speed, for this there are the electronically controlled shutters ones, the 18/19, The 212k has 1/8 and 1/15. Or in full mechanical, the uncommon leaf shutters 4/5/6, and of course the Start, not a Zenit proper but still a derivative.
For very fast lenses at full aperture in sunny days, ND filters must be used.
----------
A famous case of bad: the Almaz manufactured <10,000 by Lomo: they were instructed to make an implementation of the Nikon F2, engineers told they didn't have machine tools able to produce high precision level of gearwork, pawls, cogs, etc , for such a camera, but politicians wanted it to be done anyway. The result was a non-functional camera that did break or jam easily. Working exemplars were the ones rebuild by some outside the factory.
When I was a kid Zenits were sold new in England. Even then they were seen as the thing you got only if you had to have a new camera and couldn't afford anything else.
Trying to re-write history that they actually were good is just that.
what rewrite? when i was kid at the same time, Zenit and Praktica were a staple of the mail order catalogs Quelle/Otto/La Redoute/3 Suisses/etc. People who couldn't afford a BMW did buy a Fiat 124 and instead of a Nikon or Pentax, a Zenit. A Fiat 124 "sucks" relatively to a BMW, but it's certainly not a bad car and Zenit opened the world of interchangeable lenses SLR to tens of thousands of people who otherwise would have kept point-and-shoot scale cameras.
----
about features, early japanese SLR didn't have 1/1000s either, were knob winding and no instant mirror return.
The Miranda-D I picked last week caught my attention because its mix of similarities with a Zenit-V and a Start: interchangeable prism, rotating lift-to-set speed dial, and like the basic Zenit, a simple latch for opening the backdoor. It's better finished/polished than the brute stamping of the Zenit, otherwise not much more. A bit like an old Praktica for the slow speed setting by dedicated dial (the Start is better, it has all speeds to 1/1000 on a single Zorki-3m/4 dial, takes removable cartridges and has internal film cutter, Exakta-like). No lightmeter of course, not even uncoupled like the Z-E.
the classic Zenit was just a 1930's design at core, manufactured until the 90's...