The terminators suffer from a number of problems I'll go over each:

1) their role, terminators are very much an assault unit they need to be in close combat to put in any kind of meaningful work. The problem is getting them there. Prior to sixth edition and the massive price drops of everything with a halfway decent gun and the creation of overwatch deep striking wasn't a half bad idea. They got shot at sure but they weren't shot at by that much really and they could charge in mostly unharmed and even if they did go splat chances are the kitchen sink was what did it.

Today deep striking is basically always off the table. Their going to get brought down by relatively cheap units if they deep strike. And if they don't they probably won't ever see close combat unless they themselves are charged.

Foot travel was basically always off the table, they were too clumsy and slow to put in much work before and now they can't even do that.

This really only leaves assault vehicle travel and that relegates them to being isolated picks. Deathwing terminators and grey knight terminators are a little better at all their avenues of attack listed here but they still aren't anything to write home about

2) functionality: other armies have terminator Esque units as well but they unlike the terminators have evolved. They have been adapted to the game as it has changed and evolved over the years, while the terminator has remained the same as he always was. He's a relic of days gone by functionality wise. So while crisis suits have gone from being a damage control deep striking unit to a hit n run geurilla fighters and ogryns have become walking chest high walls. Terminators have and always will be a heavy assault unit. And there's just no use for a unit that hasn't or can't adapt to a new rule book especially one that's been neglected as much as a terminators has been.

3) investment: in a world where the meta game is all about the race to the bottom of price vs efficiency/raw stAtistics points and value have become kings. As noted above terminators are old fashioned very important point here. Back in the day your value was in equivalent exchange; how much damage you could do while you were on the table. In today's world it's all about how many points you aren't giving to the enemy when you die. It's why people rag on marines all the time. We can only trade up we can't reduce down and the terminator as the absolute worst for reducing down. He costs as much as some tanks in other armies and that's per terminator. If even one dies your basically throwing a rhino off the table. It didn't matter how many he killed or what he accomplished, that's a whole rhino. And because he dies so fast to cheap units a whole squad is throwing away five of them at one time.

Compared to meganobz or eldar wraiths that's a bloody fortune!!

So in summary here: the terminators are a melee unit you pay a kings ransom for, that die to rank and file soldiery as quickly as anyone else does (again unless in cc) in a rule book that penalizes close combat units at every turn almost, and on top of all that they can't be retooled to do something else because gw has neglected them so much over the years.

Now does this mean they can't be useful? No heavens no they can put in work and do a lot of good but they need a lot of babysitting and your army can't just leave them to their own devices they just get walked all over otherwise. Surround your enemy in problems and then introduce him to the terminators. And watch them the way the empower intended. Or back them up with lots of other cheaper units to supplement for their deficiencies.

If you can achieve that in your list building and table play they can be piles of fun.

A few days back I used just six of them to great effect against black Templar, I smashed land raiders, and vindicators, and Razorbacks, and devestators. It's always a fun time when the terminators fire up the ol pain train and start handing out complimentary power fistings.