Personally i feel the stuart is a bit of a weird tank. Its a more expensive and well armoured T70 yet its AI is pitifull compared to the T70 and has no late game potential. So the only real purpose is to use it as shock tank against less experienced players and to counter the 222 and the Luchs.
It's like it's geared super hard against the 222 specifically. The armour renders the 222 unable to penetrate it, but is next to useless against Pumas and up.
Its low penetration makes barely any difference against lights, but renders it completely impotent against mediums.
It's a worst of both worlds tank at the moment, with the anti-infantry performance of an AEC and the anti-tank performance of a T-70.