Flow rates . Emitters come in a variety of different flow rates. Most soils can't absorb the higher flow rates, so the water tends to puddle around the emitter where it evaporates, or it may even run off into the gutter. With drip irrigation you want the water to be immediately absorbed into the soil as it comes out of the emitter. The best thing to do is to buy a sample or two and test them.
Some types spit a small stream of water when they are first turned on.
To be real honest, for residential use most emitters I have tested seem to work pretty good. The biggest problem is mineral deposits which build up in the emitters. These mineral deposits can plug emitters with small openings. There are several companies that buy emitters in bulk and repackage them for the retail market. In the past I have seen different types of emitters marketed in the exact same packaging.