The biggest problem is that they're not very cheap.
Here's one.
http://www.nauticaldesigninc.com/tackle2.htm
There was a fairly long thread here about Smart Tabs vs Hydraulic vs Electric. Just my opinion but, I feel that the V20 is a little too heavy for the Smart Tabs to work effectively. The price is attractive but in the long run, I think you'd be happier with one of the other two choices such as Lenco electric trim tabs or Bennett hydraulics. There are other manufacturers choices. These are just two examples.