Well, in outboards they can be hard to replace. I've done it. It usually requires dropping the lower unit, which can be tough, with corrosion and other factors, and is not a trivial thing to do. Certainly can't do it in the water! I've only ever done it on a motor stand and in my shop. And then you need all the right sealants, etc., to put it back together properly.Then again, impellers are not that expensive nor hard to replace, if you want to run them dry, have at it.
That said, these impellers are pretty rugged, in my experience, and last a long time; like, 10 or more years. Maybe not recommended that long, but I can attest to that.