Given the way I have read and enjoyed Dan Abnet's Gaunt's Ghosts books and Brother of the Snake, and Sandy Mitchell's Ciaiphas Cain books, I had to say that yes, tie-ins could be as good as stand-alone literature.
Of course, both of those are gaming tie-ins. And ones where they were not assigned pre-defined characters. Or even assigned a particular setting within a galaxy-spanning universe. Though they do draw on a deep and complex world-building and history.
On the other hand, among webcomics, there are Rusty & Co and Order of the Stick -- D&D tie-ins, effectively. Where they don't even take the setting, just the rules (which they proceed to fold, spindle, and mutiliate at whim for comedy and plot effectiveness -- the author of Order of the Stick has explicitly decreed no character sheets for his characters, to keep himself from being locked in). This does mean they have to devise their own worlds, not just a particular setting in them, and history for them, which can be a problem in that thickness is hard to whip up in an afternoon. Then, havinag read some D&D tie-in novels, avoiding other people's worlds is not always a bad thing, because then you can avoid world-building flaws, only some of which stem from need for playability rather ability to be written about.
Though if the best tie-ins are the least bound by constraints, it does help explain the reputation of tie-ins.