One thing you are giving up with man-made versus real live rock is the porosity. One of the reasons real liverock works as your bioload is the porosity provides tons of surface area for nitrifying bacteria to colonize which will convert (toxic) ammonia into (safer) nitrates. Large pieces of liverock can also colonize with denitrifying bacteria (break down nitrates) deep inside the rock.

Regarding the other critters that come in on "real" liverock. Personally, I think that 98% of them cause problems down the road....they either die at some point or live and turn out to be a pest.

In my tank, 75% of the rock I used is ceramic made by The Alternative Reef. And the other 25% was established live rock from a former tank, to help "seed" the bacteria population. I am fairly sure my nitrifying bacteria population isn't what it would be if all the rock were liverock. However, I also have sand for substrate which also provides surface area for nitrifying bacteria to colonize. I might be a bit apprehensive to use a high percentage of man made liverock if I were running a bare bottom tank (without considering other methods to deal with biological filtration).