With Chieftain my dislikes was apart from being under powered it was the gunnery side.  When we used the .50 ranging gun it was plane and simple and a good crew could acquire a target, range it and kill it in an average time of 8 to 10 seconds. You was as fast as your loader could load.  I was a  loader for some years and on one stab battle run I didn't know I was being timed from the tower.  When we got back to the hard standing I was told I loaded at a rate of 1 round every 3 seconds.  That was fun.

We then went on to be the first to introduce the TLS (Tank Laser Sight) which we thought was the business.  We then found that the Barr and Stroud laser was a pile of donkey shit.  We could only lase 3 times a minute before it over heated.  I don't know when the M60 got it's TLS but I know that when I spoke to a Leo 1 crew the following year they had a Barr and Stroud laser that put us to shame.  Why was theirs better than ours, simple the penny pinching bastards at the MOD.  We then became the first to have an computerized gunnery system and we jump in feet first and went again for the cheapest option.   Instead of sitting back and seeing someone else run with this and finding out the pit falls, we went for an analog computer.
My Regt had the honor of being first to do the acceptance trials for this.  Our hanger first had to be fitted out with heaters as the system tended not to work properly at the start of day if the temp was to cold.  It didn't come complete because of cost so we were missing the met probes and the Commanders PRI (Projector Retical Image)  which injected the ballistic ellipse in to his sight.  With the missing PRI you could not use the OTT on tank trainer as you've guest it, you needed the PRI to use it.  At the start of us trialing it the computer dumped the program frequently and it was a ball ache to re-program it.  Having ironed out the faults when we sorted out it's teething trouble it did work bloody well.

Trained on Chieftain, worked on Challenger and Challenger 2.
Avoided the gunners seat at all costs, too cramped for me.
Loved the amount of stowage space in the drivers cab of Challenger, until we went to Bosnia on Op Resolute 1, and I had a dozer blade fitted, so I lost a stowage tray and my electric heater...
By then, the tank had its bugs worked out.
Challenger 2 however....
It was technologically better, but with loads of bugs still.
The biggest were the tracks.
Vickers specified the Leopard 2 track, so the army brought a cheap copy made in England.
A live track with a life expectancy the same as the life of the tank...
After 200km, they were shedding their track pads, after 1000km they were starting to develop fractures!
We were told to take the tracks off and put them on the other way round at intervals to increase track life ffs.
Was only practical if you had a FLRT (fork Lift Rough Terrain) licence like I did.
Think the tracks have been replaced now.