Suspension of disbelief knows no genre. The writer needs to make sure their reader is able to suspend their disbelief for the story they're telling, from the first page to the last, whether the story is about a divorced man undergoing chemo, or about vampires, or flying horses. It's about the reader 'buying' the story they're being told. And it's the writer's job to sell it.
I read a short story about a man and woman in a rowboat where the man was poking the moon with a pole. Italo Calvino told the story in a way, that yes, my suspension of disbelief was sustained throughout. I also read a story set in Guatemala where the drug lord was so poorly characterized and the plot so convoluted, my suspension of disbelief went flying out the window. At that point, I stopped reading and threw the book down. I no longer believed the story I was being told. I wasn't 'buying' it.
A reader needs to be able to suspend their disbelief to buy into any story they're told regardless of what that story is. That's how I see it anyway.