Tumblers are widely used by many professional coin cleaners. They can be one of the most awesome tools in his coin cleaning arsenal.
They are only used on severely corroded coins such as shipwreck coins and those that have been buried for hundreds of years.
In fact, Mel Fisher used one to clean many of his pieces of eight. He used stainless steel shot and water.
I've used a tumbler to help clean many hundreds of ancient coins.
The secret is in how to use a tumbler. Like electrolysis, they are used very sparingly. It would take far too much space up here to explain the whole process.
I just wanted to post that rock tumblers are definitely no joke. They are very good tools when used with knowledge.