I would have to think it's the angle of the drain that prevents the leaks if you want a curbless shower. You would definitely want a contractor who knows what he or she is doing when installing it.

Make sure the shower head points away from the door and that the shower head's cone of water is narrow enough not to hit the walls.

It is entirely possible to create leak-free showers of any type, including curbless showers (which are our favorite - very classy with great utility). Whether you use a single circular drain or a trench drain, the proliferation of set up units and floor systems means that much of the uncertainty of "how-to" is a thing of the past. Having said that, please choose tile guys and plumbers who have done this before. Hanley Wood is a supporter of ours and one of their publications, The Journal of Light Construction, had a useful article in June about fixing (read, preventing) leaks. I can't attach the article here but would be happy to provide it to any who would like to have it.

An important piece of installing shower pans is to make sure the pan is level. This prevents pooling or water running outside the shower. Then the pitch planes on the shower pan should make the water run towards the drain. With the waterproofing that goes over the pan should also prevent leakage.