They have an upgrade option "PIG Pontoons" which gives you some extra padding. You can also add foam backing behind your plates.
I wear mine every time I go shooting and it's never been unbearable, At the end of the day your hauling heavy body armor, it's not going to be the best experience of your life. For any carrier houldering a rifle is awkward, moving is awkward, pretty much everything you do wearing plates I s awkward. Additionally unless society collapses you won't ever have that carrier on long enough to think about how comfortable it is, if it has to go on your mind is going to be on real issues like the threat you are responding to.
I wouldn't say any carrier is going to be comfortable, but I can say the STT isn't uncomfortable.
I sold my Mayflower APC in favor of the STT due to the tubes system and do not regret it one bit.