Their effects can, e.g., be clearly seen in changes to the bone marrow, which IIRC has some of the fastest-reproducing cells in the human body.
We don't have long-term data yet on what this does to users of this drug.
My wife has to take such drugs as she has HIV, and I wouldn't be surprised if it ended up shaving a few years off her life expectancy. She's fine now, but who can predict the cumulative effects? Of course, since without the drugs her life expectancy would be drastically lower, she obviously takes them, and they are a godsend.
But I'm kinda waiting for the other shoe to drop.
What I'm saying is: it's great that we have this option, and it should be used where it makes sense. However don't think that this medication frees you from worry in the same way vaccinations typically do. The tradeoff is much different.