The gun was powerful and the missile worked but they were not exactly great and were overtaken by developments. I had a quick glance at wiki and the main problem of the missile seems to be a HUGE minimum range of 800 yards, on a missile that goes out to 2200 yards. So nearly 50% of your range the things unguided.
The big 'ol HEAT rounds for the 155 would be nasty, but it could only fire HEAT rounds, and these were being countered by new developments in armour at the time, and there was also issues with its propellant not being all consumed. Plus as a single fixed round its HA-UGE so you can't really carry many rounds in the tank.
The HEAT rounds only is a big problem. After WW2 and the widespread introduction of HEAT effect weapons, tanks it seemed didn't need armour, because the more armour you put on a tank, it just gives the HEAT jet more to cut through and makes it more devastating. Its why the French and Germans with the AMX-30 and Leo 1 had very very thin armour and relied more on angling and tactics/technology to not be hit, or score the first hit before the enemy (IE elements of the 1st Guards Army storming towards you) could hit back, then fall back fast to the next firing position.
But composite armour greatly reduced the effectiveness of HEAT rounds and folks had to go back to very high velocity kenetic rounds to penetrate again.
The 551's gun system was based on the MBT-70's but the barrel was shorter, meaning it couldn't fire the APDS rounds they made for the MBT-70, and could only fire a fairly low velocity HEAT round.
This meant that units equipped with the gun/missile system of the M-551 could be in the horrific position of not being able to hurt a hostile tank because of its ceramic and composite armour whilst it can very much hurt them.
