How bad are conditions on Mars if the rovers we have sent there are still functioning?
The most hazardous things to humans there do not affect machinery.
The 2 most hazardous things about Mars are the atmosphere and the lack of a magnetosphere of any appreciable strength.
The first one is pretty obvious, there is almost no oxygen there so humans cannot breath there. This has no impact on a rover. The atmosphere is also so thin that you couldn't really breathe anyway. There would be a pressure gradient forcing all gas out of your lung including any water vapor that would have to form since the space in your lungs would have zero humidity. Essentially you would dry out. This also has no effect on a rover.
The second one has to do with solar winds and cosmic rays. On Earth we are shielded from both of those things by our strong magnetosphere. On Mars the magnetosphere is very small and so the surface of Mars can get bombarded with high energy particles from the sun and even more dangerous cosmic rays (high energy protons from outside the solar system). Both of those would do all sorts of havok to a human body (imagine getting several xrays everyday).