Another point to make when it comes to navmesh agents on slopes in general, make sure that whatever mesh they're travelling on is smooth and has nothing like a vertex poking up, I have found that dramatically interferes with the agent behaviour. The pathfinding will calculated correctly, but it won't take into account small bumps like that, so the agent will blindly walk into it and get itself trapped, yes I'm aware of the being able to kind of nudge it out of the way with some code if it gets stuck but that doesn't always work. Better option is prevention and to make sure your walkable meshes are smooth surfaces.
This one caught me out pretty badly so I thought I'd mention in case anyone is struggling because I found it odd the agents worked perfectly on flat planes but not sloping hills etc. I feel like a lot of tutorial makers don't go into this and cheat a lot when they're doing "Do this" tutorials and only ever do their navmeshes on a flat plane because that is not what most game devs are going to have to deal with in real world scenarios.
Also if you're dealing with some sort of custom terrain, if you click mesh and then generate navigation mesh in the drop down menu it will generate a much more accurate mesh than what you get if you generate the navmesh the normal way for some odd reason.
