It can be somehow true at low tiers where the MBTs actually have big armor problems, and often a very long reload time with a relatively low alpha damage.
But from mid tiers, and way more at high tiers, I disagree. With the same MBT in the same mission is possible to be destroyed really fast and also to do little damage in the short time you survive, but also is possible to bounce a lot, having at the end of some games like 40-60k mitigated damage, not necessarily driving an ADTU or an Object 490, while doing a lot of damage. The reload gets shorter and the alpha higher, but also the HEAT rounds are very strong as well is strong to aim to the right points to trigger fires and ammo rack explosions. The ADTU has a very long relaod, but when the 25-30% of the damage you do is due to fires and ammo rack explosions (using the green ammo) you are perfectly fine with that slow reload. Using only the more user friendly AP and don't aiming for the right points to set fires and explosions the damage done is surely low.
The problem for the MBTs is completely on the player's side. When in a mission i see 2 players driving the same MBT and one do 30k damage and the other 12k it is very clear for me that the problem is only on the player side.
The MBT class is probably the easiest to play (PvE wise) after the TDs used as long distance snipers, where you have only to learn where to place your tank in each mission then do pigeon shooting all the time, but is good that at least needs some skill. Both to protect yourself, the armor is good only if you are able to use it properly, an offensively, you have slow reload, but big alpha using Heat and a good chance to ammo rack and damage components with AP and the green rounds. With most of the T10 MBTs is quite easy to ammo rack a T15 with 2 shots and to set fire to a Bradley or most LT with a single one, a little research with the armor inspector should teach how to any one that is willing to learn and improve, just as one of the many examples.
Imho MBTs with an armor that makes them almost invulnerable and a gun that has big alpha and pen while also reloading very fast would have been a very easy implementation for the novices, but also a boring and overpowered one.
About the mobility surely a Kornet is way faster then the MBTs, but when i drive some LT or even some AFV like the Griffin I am not running at the double of their speed, Sure with some MBT in GLOPS you need a lot of time to go to the opposite side of a map, specially if it is a large one, but I remember how when Balance 2.0 was introduced the T14 speed was buffed, it could go faster then many tanks of other classes, was fun to play, but i am happy that not is no more like this.
I feel that only at low tiers the MBTs are a weak class, slow, easily penned, low alpha and long reload, but from mid tiers on they start to become an interesting class, with its own power and that to shine needs at least some skills and an idea of how to play them. That is not climbing hills to snipe so the ones without armor have to do the dirt work for them.