I haven’t used one but I’m familiar with them. They make a lot of sense for small whisk(e)y batches where you get wood contact and some oxygen egress without the enormous loss to angel’s share of smaller true barrels. I have a used two gallon whisky barrel that I use for aging sour beer and I lose about 40% of the volume over the course of a year.
I can see the bad motivator barrels working for the same reason with beer although I’m not totally convinced the benefit for beer would be much different from adding whiskey-soaked cubes or staves to a fermentation vessel. If you’re buying one anyway for aging spirits, might make sense to use it over buying cubes/staves, too. Rotating beer and bourbon through would emulate those beer barrel aged whiskys that never really took off commercially. Aside from bourbon that requires new oak, there’s a lot of life in wood and you could do several batches before you’re going to completely lose oak flavor contributions.