Personally, I like Ms. Day as an actress and have enjoyed all her work, includingTallis.
However, where I can somewhat agree with some posters is that MOTA removed player agency in allowing a dangerous spy from a hostile power to escape with vital information - and there was nothing my character could do about it. While I personally did not want to kill her, I could easily see why some characters would have done so - and I think it was a mistake to make her "plot proof."
I suspect that someone wanted to leave the option open to bringing her back in some future DLC or something so prevented what should have been, in game context, a reasonable action.
The same thing happened with Flemeth, Morrigan and Leliana and personally, I don't like it. If the writers set up a situation where someone ought to be killed, that person ought to die. If they want that character to play some important, future role, then create a believable reason why you can't kill them. I do not think the way Tallis was handled was believable.
And one final comment; please, do not put Ms. Day in the same category as that "person" in ME3. Felicia Day is a talented, professional actress with a solid resume of respectable acting credits. You may not have liked the character, but there was nothing wrong with her performance.