Allergic reactions is a main issue with the L&P system, due to the allergens contained within the monomer liquid. It must be handled carefully and you must avoid any skin contact with it. Care must be taken to always achieve a proper cure or else the client will be exposed to the allergens during removal. 

An important aspect of using the L&P system is the mix ratio, that is the amount of liquid monomer to polymer powder that is used. Too much or too little polymer powder, or too much or too little liquid monomer, can create a nail coating that is too weak or too brittle.

There is another type of liquid monomer used in some budget salons, and that is MMA. Unfortunately this has terrible nail adhesion so the nail plate has to be deeply etched to make a suitable surface, MMA is overly strong and too inflexible meaning it can break off taking nail plate with it, it is also harder to remove. EMA while more expensive than MMA is the better choice in the long-run with less chance of losing a client due to the damage inflicted upon their nails.