Not too bad but rust can cause a bit of a problem.
Not when removing as the old unit just needs to come out and you don't have to be delicate.
But a really good clean of the hole ensures the new unit goes in clean/smooth.
You will need it scanned to tell you what sensor is shot.
And buy the best you can afford,
Last year I fitted two owner supplied sensors and one was faulty from the box,
Other car came back in within a month for the same thing.
Both were cheap parts,