That's just it. Specifications are guidlines to ensure the equipment will give the user the performance expected, however, they are never absolutes. Take ethernet over Cat5 with a specification of 100m for a segment length (node to hub/switch). At 101m the signal doesn't stop it is still there and most likely completely usable by the end user, however, the performance of the link might be degraded a certain percentage.
Exceeding specs (regarding performance) if required is usually worth a try to see if it will work, however, you cannot complain if it doesn't work as advertised and be glad if it does (dmoore's example).
As far as wireless vs wired, I'm a firm believer it is always 'cheaper' to run a cable (if possible) than to use wireless. By cheaper I'm including initial hardware costs as well as configuration time, performance levels, troubleshooting issues, maintenance, lost productivity due to down time, etc.
I'm new to this light automation and will be getting a wireless setup to test and hopefully use but I doubt I'll rely on it for the whole setup.