Simply put:
Don't purposely void the warranty on any optics if at all possible.
Stanley's was the first (and still is our only) Mueller scope that has ever leaked water, but it so far is the only one we know of that was intentionally dunked for dipping as well, at least that we've sold. The process to dunking is more than just spraying water on it such as would happen in a rain storm.
We've sold hundreds upon hundreds of Muellers, so the sample group is pretty large.
Mueller scopes offer great bang for the buck in optical clarity. If you want a better no fault warranty, then go Vortex or Leupold. Mueller's warranty is no different than many other optics companies. Vortex's stuff is a little more beefy, with a better warranty, at a slightly higher price point (IMO) for the glass clarity, over the Mueller's.
Decide what price point you want, and then pick the best glass, reticle, and other features in that price point you want in the brand of your choice. You'll never get a VX4 scope out of a VX1, either, so don't expect to in any other brand. IIRC, the Leupold VX1 scopes have mushy clicks and aren't even fully multi coated?