Connecting single-mode and multi-mode transmission in most cases requires using active OEO media converters, which detect the optical signal from one type of fiber, and then regenerate it with a laser optimized for transmission in the other type of fiber. This of course means higher costs of such a solution, and also limits it applications to places with available power supply.
The neccessity of using active media converters arises mostly from the differences of single-mode and multi-mode fiber core diameters (about 9 um and 50 or 62.5 um, respectively), which determine the numer of modes transmitted by the given fiber type. Especially in the case of transmission from MM fiber into SM fiber, it means very large and randomly changing optical losses. The attenuation of such a link has the value of a few dB to above 20 dB, depending on the light source used (LED, single-mode or multi-mode laser, VCSEL) and, even worse, it can change randomly and abruptly by a few decibels.