the soviet t34 itself worked on similar idea as a medium tank. The kv-1 itself were also plagued with various problem. Is-2 was actually reliable but joined the war late.
the t34 and sherman were vehicle built within the technology capable at the time. The germans had to stretch the technology to make the panther.
and ironically the German reached the zenith of its expansion using the panzer 3, a medium tank. The invasion of France was done mainly using the pz38t.
They didn't start using the tiger and panther until they were losing.
Where do I even start?
This was the number of tanks used in the attack on france:
Panzer I : 523
Panzer II : 955
Panzer III : 398
Panzer IV : 280
Panzer 38t : 228
Panzer 35t : 118
So NO the main battle tank of the attack on france was NOT the Pz38t but Panzer II and III.
https://de.wikipedia.org/wiki/Westfeldzug#Deutsche_Panzer
Now regarding the T34. It was just as unreliable as the Panther/Tiger. More than 50% of T34s broke down with mechanical failures in the first months of their service. They even had to carry a additional transmission around because the transmissions were so poorly designed.
"Our armored forces and their units frequently suffer greater losses through mechanical breakdowns than they do in battle. For example, at Stalingrad Front in six days twelve of our tank brigades lost 326 out of their 400 tanks. Of those about 260 owed to mechanical problems. Many of the tanks were abandoned on the battlefield. "
"Soviet tests on newly built T-34’s (15) showed that in April 1943 only 10.1% could complete a 330km trial and in June ’43 this went down to 7.7%"
http://chris-intel-corner.blogspot.de/2012/07/wwii-myths-t-34-best-tank-of-war.html
The Germans also didn´t lose the war because of the Panther. They lost it despite the Panther.