If you use the correct materials it will be problem-free. 99% of the rock in my tank is manmade.
Search for "aragocrete" or "MMLR" on the forums and I'm sure you'll find plenty of reading material. The basic approach is to mix portland cement with various aggregates (sand, crushed coral, etc.) to get the texture you want. Then you just mold it into the shape you want and let it cure. The cement is mostly hard in a few days, but not fully cured - you need to cure it fully before putting it in a tank or it'll spike the pH.
Traditionally, manmade live rock is cured by soaking in water and changing the water every few days for 6 to 8 weeks. Smaller pieces, like frag plugs, can cure much faster. You can also leave it out in the air to dry for a few months.
If you want to get the "traditional" frag plug shape you can drop spoonfuls of the cement mixture into dixie cups, then tear the cup off once it is cured. Other people use egg cartons or other forms. Or you can just blob it into depressions made in a tupperware bin full of moist sand to get random shapes. If you want to get really tiny plugs suitable for a zoanthid or two each you can use eggcrate.