Hi,
SNMP has public directory that allows anyone monitor CPU, interfaces etc. Usually you can use any network management software to monitor those basic informations.
However, when it comes to nice stuff it is not that simple. We can say that there are hidden SNMP directories. For exame, let's say you try to monitor a WLC using Orion. Probably you'll not get all the information that you get when use Prime even those when you get the whole WLC MIB. There will be always some OID that does not work.
Didn't try SNMP with APIC-EM yet but I wouldn't expect too much with third-party tools.