Medium tanks are generally fine, the main problem is that heavy tanks arrive so fast that there is often just no use in investing in medium tanks. Especially when you already have a vetted LV around that survived early to mid-game. Take OKW for example, you can get a Tiger around 3-4 minutes after the P4 when you have a decent fuel income. Unless you are struggling hard going straight for the Tiger is often just a stronger strategy (maybe excluding 1v1).
The only medium tank that I really don't like is the Cromwell. It just has no role at all at the moment. T34 is cheap, spammable, strong vs infantry and can ram to finish off tanks with off-map or in combination with SU-85. Sherman has HE rounds that rape infantry and the bulldozer upgrade that can turn it into a premium medium doctrinally. OKW P4 is a premium non-doctrinal medium tank with very reliable main-gun and high armour that makes it hard to take out with Cromwell, Sherman or T34. Ost P4 has the same extra armour at vet 2 and is a good all round tank. Cromwell has slightly better mobility but that's just not enough to compensate for how mediocre everything else is. It needs some sort of redeeming ability.
