And it counters lights quite well. I'm not really sure if it is meant to fight mediums as a primary role, and its current abilities and sight range with veterancy allow it to support allied medium tanks or tank destroyers quite well without any penetration buffs or smoke dispensers.
I will freely admit that I have never really seen this unit as a Puma/AEC analogue.
The Stuart used to be a generalist like the T-70, and a wildly overpowered one at that.
The first community balance patch smashed it hard with the nerf hammer. They removed the movement stun from Shell Shock, raised its price, slapped a hefty damage malus against infantry on its main gun, reduced the accuracy on its MGs, and gave it +10 penetration in return.
And thus Captain became a dead tier for a year.
The USF tech revamp brought the Stuart back by putting it in the same tier as the Utility Car. It's decent against lights, fast enough to keep up the Utility Car and more importantly it's your only decent AT option in that tier.
But after that tier, it's next to useless. The AEC and Puma can support a Cromwell or Panzer IV against armour. The Stuart cannot do the same for a Sherman because its penetration caps out at 75.
If it's going to just be a platform for Engine Shot and Shell Shock, it should at least be able to survive the attempt.
The smoke simply isn't needed. Most other LVs get away without it, so I don't see why the Stuart would need it, either.
As for the Skillshot engine critical, I really like the idea.
Anti-infantry lights don't need it, as they're for fighting infantry.
All other anti-armour lights have smoke dischargers or 60 range so they can survive post-light vehicle phase despite their brittle chassis.