The 152mm fired a righteous HE shell, and a fearsome flechette. Both low velocity though. The biggest issue was the loose caseless propellant, which broke up, and represented an insane fire hazard. Also slowed ROF to 2 per minute, compared to a good M-60 crew's 9 per minute. The other issue was the recoil - the aluminium hull was so light, firing the gun rocked it off the front of its tracks, and knocked electronics (like the missile gear) out of its sockets.
The Shilleleagh (sp?) was also a step ahead of the bleeding edge, with significant reliability issues. It also had problems targetting within 800m, not the most uncommon issue with first-gen AT missiles (see what I did there?) They tried it on the M-60A2 "Starship", but failure of the Sheridan seemed to doom this.