Hi Steve, welcome to the forum and the wide world of HHO.
Can't really think of any general ones, maybe there are vehicle specific advantages?
You will find it difficult to use a PWM if you need/want to.
The chassis of a vehicle often isn't a reliable, low resistance connection at the current levels we tend to use.
OK, there's nothing "wrong" with doing that if it suits your particular application, it probably makes for a less versatile reactor though.
Did you have a particular reason for building it that way?