So you pretty much are just comparing injected co2 vs liquid carbon, just with the liquid carbon set up on a automated dosing system?
Liquid carbon (excel, metricide, glute)
PROS: much smaller chance of "over dosing" to the point of killing fish, doesn't lower pH
doesn't off gas like co2 so you can have high surface agitation for o2
CONS: it not nearly as effectively utilized by plants, as co2 is
in the long run, it's not as cost effective compared to co2 growth results
it can actually harm some plant species
Just to name a few. Injected CO2 is pretty much the opposite of the pros and cons of liquid carbon, besides the initial costs and other details to possible malfunctions of equipment I left out.