Installing from the source normally means to install from the tar ball ie tar.gz file. You would then do the whole ./configure & make & make check & make install. The package manager and Yast are sort of kind of the same thing, someone will probably get angry if I say that ZMD and YaST update are the same thing.
Anyway ZMD and or YaST update will get the lastest version on that repository that it's looking at. So it might not be the lastest version of the software.
Risks, well I guess really just that it's not the latest patch or version. Depending on what it is that your installing dependant packages either will not work or won't work fully.