For the uninitiated I will begin with a basic stat block. These comparisons are against all current non-premium Tier 3 Self Propelled Guns.
Tier III U.S.S.R. Self Propelled Gun
HP: 150 (lowest in class)
Crew count: 6 (highest in class, tied with M7 Priest)
Speed limit: 30km/h (lowest in class)
Engine: 130 HP
Power to weight ratio assuming full load: 9.42 (lowest in class)
Turret: 12'/sec (no comparison)
Combined traverse: 34'/sec (easily best in class)
Armor: 15/15/15 (Comparison hardly matters)
Radio range: 615m (Further than you can actually shoot)
Your main upgraded weapon is as follows.
122 mm Howitzer mod. 1930: Tier 4 gun
Rate of Fire: 6.74
Overall damage per minute: 1011 (worst in class)
Sometimes tanks give you a selection of weapons to fit your play style. Artillery usually doesn’t. At first glance the 76 mm Howitzer mod. 1903/30 might appear to be the better weapon despite being a Tier 2 gun. It clearly has more ammunition and despite being Tier 2 somehow has a higher damage per minute output. Don’t be fooled. The slightly lower penetration doesn’t seem terrible but the range is. The 122mm has a terrible range, make no mistake, but the 76mm barely hits to 500m. With the 76mm chances are if you’re in range to hit a target, they will see you as soon as you fire. It’s not worth it.
IF IT'S SO BAD, WHY AM I WRITING ABOUT IT?
Numbers aren't everything! For starters, the SU-26 is one of only two Self Propelled Guns with a full rotating turret. This is definitely considered an easy-mode artillery piece and can even be played well with one arm. The full turret allows you to beat out all other Tier 3 Self Propelled Guns in gun traverse and allows you to maintain stealth while changing aim more than a few degrees, if you can tolerate the lethargic turret traverse speed without turning your hull to assist.
This is a distinct advantage, yes, but somewhat easily mitigated by experienced play. Many complain about the very rapid rate of fire and this is also very nice, but is balanced by relatively low shell damage and overall it does rate at the lowest damage per minute of all the Tier 3 Self Propelled Guns.
Ultimately though, I believe the reason people say it’s overpowered is because they die to it a lot. Victims will always look to the number of kills to cite what they consider the most overpowered tank on the opposing team. The fact of the matter is whoever lays the deathblow is credited with the kill and a fast firing artillery has more chances per minute to lay the killing blow than competitors.
People also cite the high arc of fire and yes, the SU-26 does have a very high firing arc allowing it to land shells closer behind hard cover than other Self Propelled Guns could. It also has a fast aim time and short shell travel time due to short range. These factors do at least seem to help it hit more often than not. The balancing factors here are how close to the enemy you have to be and how little penetration the shells have.
Being a Tier 3 Self Propelled Gun you will see yourself pitted against up to Tier 6 Heavy Tanks. Here is really where higher penetration and alpha damage artillery beats you. The penetration and damage are plenty to ruin a Light Tank's day, especially a low tier one, but you might expend more than half your ammunition supply over the course of several minutes on a single KV-1 and not kill it.
SHOOT ALL THE GOLD
Premium ammunition isn’t terribly expensive for the SU-26. It will one shot lower tier tanks with alarming frequency but it is not without “issues”. Since the introduction of spaced armor you will frequently get zero damage penetrations with premium HEAT shells. This all depends on where the shell hits but you can expect zero damage penetrations from armored track skirts, upper track guards, and spaced armor segments on most Heavy tanks. I have seen shells go straight through a KV-1 at freakish angles for zero HP in damage. HEAT shells also have no blast whatsoever so near misses will always do no damage and will not even have a chance to damage the tracks of your target unless it impacts them directly. For these reasons I no longer load them. Your mileage may vary of course.
HOW'S THE DEFENSE THEN?
The defense is actually one of the SU-26’s better points. Due to your short range you will be on the defensive frequently. You will not outrun anything, so trying to get away instead of moving to get terrain between you and your attacker is futile. You do have a very low profile though and while you are unlikely to bounce anything off your hull you are more difficult to hit than most artillery. Unlike some others you can level your gun quite well for direct fire and are likely to kill your attackers if they’re not quick on the trigger. This is assuming they’re not three tiers higher than you, of course.
Your stealth is good due to your small size and not being required to turn your hull to aim, though your turret rotation is quite slow without it. Keep in mind though, you do generate tracers like everyone else, and just because they can’t see your outline doesn’t mean they don’t know which bush you’re hiding behind. Your turret, however, ensures that unless someone does have you spotted they won’t know what direction your hull is facing and therefore what direction you will actually move after firing making counter battery work far more difficult. Again, your reload is quick but do be sure to move after firing even if it’s only a few dozen meters.
WHY DO PEOPLE LIKE IT SO MUCH THEN?
It’s a Troll tank. I love Troll tanks, and so do a lot of other players. The fast reload and quick aim time means if your crew is well skilled with Sixth Sense, Snap Shot, Smooth Ride, and Camouflage skills you can rapidly turn your attention from tank to tank laying a truly withering hail of fire on them. If you equip a Camouflage Net, Artillery Shell Rammer, and Enhanced Gun Laying Drive people will find your speed and accuracy hard to believe and thus it must be overpowered.
What most of it boils down to though is the SU-26 is a Tier 3 Self propelled Gun and given the low tier, will get many opponents with very few games under their belt. I feel it is that lack of experience in your enemies that accounts for the success of the SU-26. At those tiers most people haven’t learned the best ways to hug cover, angle themselves, and jink while driving to avoid incoming fire. When the SU-26 player encounters someone who does these things well, they switch to a different target because there are plenty of others who don’t.