This seriously depends on what you mean by 'custom mod'.
If you mean a plastic box with some
batteries and a feeder in it, yes, there is a MAJOR difference beyond aesthetics.
If you mean strictly function...well, yes and no.
Does this custom PV have the following:
LED Readout
Variable/Regulated Voltage from 3.3 - 6 volts
Atomizer Resistance Meter
Battery Level/Voltage Meter
On/Off mode
16 second cut off
Short Circuit/Atomizer Protection
Reverse Battery Protection
Battery Monitoring/Over-Discharge Protection
Amperage Limiting to 2.5 amps
Stock Warranty: 1 Year
The Provari DOES have all this, and a whole bunch more....
Do you need/want all that stuff? It's not 'essential' for a very high quality vape by any means. It IS there however, and it is all very nice stuff to have built right into the PV in a small micro-board smaller than a quarter.
When it comes to how well the device is built, and the grade and quality of materials involved...there is probably a LOT of difference depending on what 'Custom Mod' you have in mind.