Fact is, vib tumblers are about as simple as possible, a plastic bowl and base, 4-6 springs, a power cord and a motor. None of the "makers" make the motors, they all buy them from the lowest cost motor maker so the tumbler makers have NO control over how good any given batch of motors may be. Most last a long time, some exceptions last a very long time while some others stop pretty quick - just luck, not the brand, may as well get an inexpensive one and hope.
Think they will all stand behind them for at least a year, some maybe two. If you get a really poor one, that's enough time to find out. Usually.