I've seen several people post this. Yes it has a massive amount of hitpoints (1280-1440), but the armor is nothing special. 220-242/110-121 front/rear armor is bad for a heavy vehicle. For comparison, the AVRE has 290/180 armor.
When doing balance comparisons/reviews, its armor is definitely not a factor to take into consideration. It won't bounce much. Its durability comes mostly from raw health.
While I agree that the armor value itself is nothing special, it should be taken into account though. Contrary to all other tanks, ATGs don't contribute much to the overall damage. 220 armor for the ST that forces ATGs off the field is different than 220 for a normal tank where the ATG can stand steady under fire.
The problem (with both ST and AVRE) is that the only unit to reliably penetrate is your TD. Mediums are very unreliable (about 45-55%, depending on range and ST vet), ATGs can at the very best only get one shot off before they need to run, but often this one shot already means that you will eat the shell that probably wipes all your vet.
The high health pool means that you won't be able to get the firepower together to kill it in time. You need at least 8 pens to kill it, which will probably translate to 10+ shots from TDs/ATGs. To get this volume of fire, you'll need at least 3 units to have a chance. The ST currently forces a big overinvestment into AT for its price.
What makes the ST broken in my opinion is its veterancy. A vet0 ST might still be okay, but at vet it loses almost all its drawbacks. Normal speed becomes good speed that will make it hard for your TDs to catch up. Mediocre armor becomes decent armor that gives even more trouble to your mediums. Normal range and sight becomes very good range and sight, very good health becomes excellent health. And it vets quite fast overall. A vetted ST is very, very hard to kill unless your opponent throws it greedily, especially given some issues as aiming while still decelerating etc.
The AVRE is a good unit by itself, but both the factions and the commander restrict it well enough to not dominate as much. Mostly because of 35 range and the lack of both a real AT brawler (Comet is decent, but does not complement as well as the Panther) or a fast firing TD like the JP4.