I think there is a key difference in the way that the EMH was originally programmed in comparison to other holograms that allowed him to develop sentience. Standard holograms can be unaware they're not real, their growth is limited by their programming and cannot make decisions that contradict their programming. Over the course of Voyager the EMH's personality grew and changed and he reached the stage where he wanted to be more than a Doctor and step outside the sickbay so he gained sentience.
But didn't lots of ships get a similar EMH (later discarded down the mines) and presumably other establishments used them for certain work. You could very easily argue (and I'm sure the doctor would) that all holograms have the potential to become sentient. Given the right programming, the right environment, the opportunity etc
Where will that lead
Holograms in comparison could not be allowed to be sentient by any degree to avoid repeating the missstep the Federation experienced with the exocomps.
Limiters.
If I program a hologram to repeatedly say (in a convincing manner) "I am alive and deserve rights", at what point do you stop saying...it's programmed to say that.....and start saying......does he really mean what he's saying ?
I simply don't accept that he's anything more than what he's programmed to think he is. There's no real personality there, no real individuality.
People keep saying he developed, changed, became more, exceeded his programming but honestly, if you listen to the doctor in season one and listen to him in season seven, is there really a massive difference.....are they worlds apart
He developed what B'Elanna allowed him to develop
He's sentient, he shows (increasing) self-awareness of his existence and purpose, and he shows some degree of advancement in character
But does he?
See bold above
Yes all holograms with the correct programming have the potential become sentient.