Tactical Terminators are like an embodiment of the space marine in terms of their identity on the table top. They have shooting power, mainly effective against foes weaker than themselves. They have the ability to take hard hitting weaponry, but at great cost. They are durable monstrosities to some, but nothing more than an annoyance to others.
Back in the index days, I used tactical terminators to great effect. I brought them in every list and they always accomplished the task and tied up more of my opponents points than they costed themselves. However, as codexes have been released, more factions getting more access to cheap and powerful weapons, abilities, things that deal mortal wounds, etc, terminators of every kind have suffered.
I could write a whole essay about it, but simply put I think terminators in what they're supposed to do is an impossible concept to balance correctly. You buff them a little bit, and they're too good. You nerf them a little bit, and they're sitting on a shelf. The concept of the terminator is just too... I don't know what the word for it should be... extreme?