With true line of sight rules now, there isn't really a lot of terrain that will actually cover a stegadon... Might make it harder to hit for some things, but even most forests can be seen through. What are people using to cover up the steg?
Artillary is indeed the main reason not to take them though, they will naturally be the first target due to their size and things like cannons and warp lightning cannons really are powerful and accurate now. They are a very nice model though, and not a useless choice, can still have a good effect on the battlefield, just need to use them a bit more carefully. Unless you are playing competatively, your army won't be at a big disadvantage for having one.
I find a plain ancient steg to be the most useful choice now. Add a chief and it costs too much, same with a priest, also puts a lot in characters.