Saftey is a big issue because a single drop of liquid propane can destroy an engine.
As to regulation Vapor Phase systems are infinetly easier to regulate. The Powershot 2000 system provides a proportional ramping up effect, as boost increases so does propane flow, liquid phase systems that do not increase flow proportionally rather they increase incrementally, they have multiple jets that come into play as boost passes thru specified trigger points another jet comes into play each jet functions at a constant flow rate, instead of a smooth ramped progression you get a stair step progression in propane flow.
Installation of my Powershot was very easy the biggest problem was mounting the tank but that problem is shared by both vapor and liquid systems. If you choose to mount the control switches in your dash, as I did, that also increases the difficulty of installation. Doing that was a matter of personal preference I just didnt want to use the supplied mounting bracket.