in an extremely general way, the answer is Combined Arms.
There was a time when t1 and t2 each cost 50 fu. There was a time when t3 and t4 each cost 120 fuel. you were supposed to pick t1 or t2 and t3 or t4. Now, t1 and t2 are 10/20 fu each, possible to build both of them and t4 requires t3.
yes, cons do not hold their own but its ok because SoV is supposed to be extremely combined arms design wise. and as you said, all the other units are well capable.
This is very true. However it is also my largest complaint regarding balance. SU requires more APM and micro to be as successful as other allied factions with the notable exception of piat infantry sections. (at least until the next patch)
The maxim, while quick to set up, must be watched at all times to prevent flanking due to its small firing arc. Cons are jack of all master of none. Zis guns cant retreat and cant cloak unless you pick certain doctrines. By necessity, T34s must be used large groups to be sucessful. SU has no sector wide offmaps and its rocket artillery is cumbersome, awkward, and slow to deliver its full payload.
This is why I play SU however. It is always a challenge.