With Juniper equipment topology generation is somewhat broken -- on the L2 equipment the MAC address information is stored at the unit (.0 or vlan) level rather than the physical interface level, and doesn't make use of the LLDP information.
this means that if you want the error stats you have to monitor the physical, and if you want topology you have to monitor the units, and if you have trunk interfaces you're stuck. If you have MPLS or VRFs <span style="NY">forget about it</span>
rob.hock: if development ever want to improve juniper support y'all are more than welcome to ask for snmpwalks or come up here and check it out for yourself.