It becomes an issue when the wire run is too long, and/or the wire used is of insufficient gauge, and/or too many devices have been installed on that circuit. On a NAC, device(s) at/near the end of a circuit may not operate as intended, or at all. The system may show no troubles while not in an alarm condition. On a SLC that is similarly overloaded, you are likely to see intermittent troubles that would take me many paragraphs to explain coherently. The Cliff Notes version is digital signal degradation due to mistakes made on the voltage drop calculations or an installer that didn't follow the drawings or someone adding devices at a later date without paying attention to circuit limitations.
It is possible to have a system that meets the manufacturer's requirements but does not meet the (in some cases) more stringent requirements of the AHJ, who has the final say on whether a permit gets signed off or not.