Also check out known bugs as tracked in GitHub for the details.
They do draw in a windowed simlulator session, no matter how large that window is, even if it stretches across multiple monitors.
If you apply a Roll rotation offset to your main screen in your Graphics settings, then you will notice that aircraft labels align with the screen, not with ground, ie. labels appear tilted. This might be especially noticeable in a networked setup, in which the XPMP2 Remote Cient synchs traffic to networked computers, which drive External Displays.
The labels appear in one eye only. Funnily, some say the right one, some say the left one...but it is consistently one eye only. This has to do with the special way text is rendered in the 3D world, which is simply not meant to use text. Text is a UI thingy, not a world thingy.
If you prefer not to see labels under these conditions in VR views, then there is an A/C Label Setting, which allows you to switch off labels in VR views.
In some situations LiveTraffic is the cause of glitches in cloud and other particle renderings. LiveTraffic does not draw clouds or particles itself, but the CSL models in operation by LiveTraffic might at least use the particle system. Issue 122 tracks findings. As it never happens to me I can't really track it down.
Possible workaround: Users report that the glitches stop when deactivating TCAS, e.g. by deselecting "TCAS controlled" from the LiveTraffic menu, or permanently by setting AI/TCAS on request only in the Basic Settings. Noone can yet explain how problem and workaround relate...but I have positive reports from 3 users.
Users report crashes in plugins of the TBM900 plane while running LiveTraffic with TCAS on.
Workaround: Switch TCAS off.
This is confirmed not to be related to how LiveTraffic deals with "empty" AI slots (i.e. temporarily not used positions of the up to 19 configured AI planes). It also happens if LiveTraffic delivers just a zero position as X-Plane would do. Also, similar crashes occurred without LiveTrafic at all but with X-Plane AI on. So ultimately, the TBM900 needs to be fixed.