Outlast Whistleblower is definitely a level up in terms of the main character development. Protagonist is not just a flat blank person that simply wants to live and nothing more (basically main Outlast campaign protagonist was not a character at all). Vice versa as the Whistleblower story develops protagonist evokes sympathy more and more with every hint on his life. That's nice I'd say, it raises the level of immersion.
As for gameplay it's quite boring and repetitive. Mostly because it copies main Outlast campaign gameplay and adds nothing new, nothing new at all which is kinda strange.
Level design (visually) and sound design are both top notch. Levels are a bit more frustrating and labyrinthine and that's annoying when you have to sneak by some enemy but it's damn too unclear where to go and you just circling around and dying over and over again. Also there is too much of blood everywhere, it simply becomes somewhat decorative not even freaky. I mean there's even more of blood in comparison with the main Outlast campaign. Anyway visually Whistleblower is a pleasure. If all of the levels were completely empty it would be still a pleasure to have a big long walkaround.