....But I don't see a benefit in specialized smoke like e.g. the Churchill's smoke giving additional buffs to infantry in it (although it would make sense gameplay wise as an infantry tank), then the smoke of an Scott giving different buffs, mortars again having different buffs etc. Even if there are reasons - different payload size, different chemicals etc - the game would not be readable or intuitive.
Funny that you mention Churchill's smoke because it the current implementation that is not readable or even intuitive. It current implementation is a stock, weird trailed "Panzer Tactician" with a different mechanic in on the most durable tanks in game. One is created by smoke canister on the turret of tank the other form the exhaust.
The name "Infantry Support Smoke", the description "A line of smoke is released behind the Churchill, allowing supporting infantry to advance safely. The Churchill will have reduced speed while deploying smoke" and the role of the vehicle all indicate that Relic was planning to make Churchill's smoke provided bonuses to infantries but was lost to a oversight.
The benefits of specialized smoke are clear. Can be used to make units better in the role and have added utility. In the case making Churchill provide cover status it would increase the synergy of the vehicle with infatry and be helpful in heavy infatry build.
If abilities like smoke in this case provide similar benefits the units bring less to the table making them less disable to build and usually the unit with more DPS is picked.
The overall idea begs the question where you set the cut offs. Even handheld grenades had different sizes. Then there is the 75mm Scott, 81mm mortars, 105mm howitzers, 120mm mortars, 152mm howitzers all using different shells, payloads etc etc. How many classes are necessary, where are the cutoffs?
Think I have provided a very specific proposal, other may add or subtract as they see fit.
Keep the system simple and understandable:
E.g. All smoke blocking sight OR giving the same set of bonuses would be okay, just the size of the cloud is different. That's an easy system. The best one for deep gameplay? Probably not, but understandable and -for the most part - intuitive. At least as intuitive as almost any other system that you want to represent in an arcade game.
(edited for clarity and added points)
I personally do not like the "shot blocker" effect of smoke and I would rather have it replaced from all smoke weapon.
Although I do agree that there benefits in keeping the "simple and understandable" in my opinion the benefits of customizing the effect to specific categories outweigh the benefits of keep the system simple.
The small arms system was very simple and the change to weapon profiles made more complicated. It also improved the game drastically.