I agree (albeit there might perhaps be a use if a MG team needs to be moved quickly from A to B and there is suitable cover). The desirability of having tachanka's acting as horsed assault weapons in a Soviet army stems from their legendary status, akin in Soviet mythology to the role of the English longbow in British (after all no one, I suspect, is asking for a similar set of rules for the German If5, ironically a vehicle far better suited to this role). However, as always, when you introduce mythological beasts into what is attempting to be a realistic ruleset, you will have problems - as you have quite correctly noted earlier.So in all I do not think they are suitable for CoC sized engagements however we all want our toys so lets make rules for em I say!
I agree - the shooting part is easy. Your chances of hitting anything smaller than the side of a barn, while stationary and at useful combat ranges, are statistically insignificant. The chances of hitting a barn are probably one in three. If the tachanka is moving at speed, the chances decrease accordingly. The vulnerability issue is also, despite appearances, easy. Either you stick to realistic rules and the thing is reduced to dogmeat and matchwood in short order (there is a reason why chariots went out of fashion long even before machine guns and QF artillery appeared on the scene). Or you equip tachanka units with, in effect, magical properties.The shooting part is easy but how to make them vulnerable to return fire without making them completely useless is harder.
Personally, I like both, so please don't take this as a criticism of anything, rather just an attempt to clarify the issues.
My comments regarding motorbikes and trucks merely relate to the tachanka's vaunted ability to support charging cavalry. I would suggest that regardless of terrain unless it is as even and vacant as a football pitch, when going flat out, the single horseman, like the motorcyclist, is going to outpace the much bigger and more complex contraption. Of course the more cluttered the terrain, the greater the advantage of the single horseman or motorcycle over its bigger cousin. As you say, the tachanka evolved in wide open spaces but also, it needs to be remembered, for warfare in which artillery was either rare or non-existent and even machine gun support was scarce. Even so, I would hazard a guess that the MGs would be deployed from aboard the tachanka only in dire extremity and if the enemy obliged by attacking with cold steel - something that occurred surprisingly frequently both during the RCW and the Polish-Soviet war (so much so that in some Polish cavalry regiments there was a return to wearing makeshift chain mail aventails!).
Talking of film references, you can find quite a lot of footage of the real thing in Soviet propaganda films of the invasion of Poland in September '39.