Most cheap converters are made from v6 or 4 cylinder cores which makes them smaller and stall at a higher RPM when put on a v8. Their cases and internals were designed for lower HP applications.
Here you can compare this cheap coverter
Here's a PI vigilante converter
As you can tell the Rev max converter Looks like they've welded on a ring to the converter so it will fit the bolt pattern of the V8.
Compared to the PI converter which is made custom by PI not by using a 4 cyl or v6 core to make the converter.