"The opinion of someone has nothing to do with his ladder position."
Of course it does. People's balance opinions are usually informed by their ingame experiences. At very low levels the MG42 absurdly powerful: players don't know how to flank them effectively so their main weakness (fixed firing arc) doesn't come into play. It's as if all enemy MGs have 360 degree firing arcs.
If they're the type of player that's inclined to believe the game is unfair instead of believing they're not playing optimally then that experience will lead them to conclude the MG42 is obscenely overpowered.
Take your specific example,
"SU-76's XP value is too low"
Take a player that's bad with the SU-76. When they use it they don't react fast enough to enemy AT and it dies. As a result they play overcautiously and it never really kills anything. As a result, they respond to your opinion with "No, the SU-76/M vets up too slowly." How do you prove them wrong?
Yes and no. Of course, you are less likely to have the "right" opinion if you play exclusively against the AI for example, but the opinion needs to be evaluated on the merits of the arguments, not on the player's ladder rank. In real life this is called a strawman argument and is a well-known logical fallacy.
In other words, if a player that exclusively plays against AI has the exact same opinion about balance than a top 10 ladder player, is his opinion right or wrong?