Olive will not have a data-plane... as a matter of fact if you watch an Olive boot, it will look identical to an MX or M/T series that has the switch-fabric boards removed as they will all boot into "olive mode" meaning 'dataplane-less' Junos routing-engine.
All those factoids aside, what it means for someone that is trying lab exercises is that things like the following will have limited or no functionality:
- Any form of traffic encapsulations (GRE, IPoIP, IPSec, etc) will not be supported as dataplane resources are required to perform these encap/decap functions
- Class-of-service: there's no switching fabric and are no PFEs, therefore no queues, schedulers, etc. therefore no CoS.
- Security services: This is more of a function of vSRX than vMX, but again, all security services would not be present in Olive as there's no FWDD to support it/them.
- Multicast: since multicast replcication is handled in the dataplane, mcast functionality is limited in it's realism though it is working in Olive. For example: for OSPF study, mcast functionality in Olive is there, but if you're looking to model realism in mcast data-plane, it's not there.
Control-plane functionality is all there. So if you're looking to study for any JNCIA/JNCIS/JNCIP, Olive will be very useful but keep in mind that vMX, vSRX and vQFX all have evaluation licenses that with a simple VM snapshot that both ESXi and KVM support, you can extend the longevity of that license with relatively little additional hassle and you've got the latest version of Junos to work with and full feature sets.
These are the things I can think of off the top of my head but you can google "Olive Juniperclue" and I believe there's a more complete list of what does/doesn't work there but hope this helps!