Displacement maps in VR
Avoid. If you really need the extra geometry, just add the extra geometry instead of using one of the most costly material effects in all of CG 
Particle systems
I use a couple particle systems in my current VR project, namely a muzzle flash effect (combining smoke and sparks) and a cloud of moths for light sources (a single orbital particle effect). So yeah, traditional particle effects are compatible, but I haven’t used Niagara effects in VR yet so I’m not sure how that goes in terms of performance.
Moderation is key! Have a go and see if the effects you want to implement incur a performance hit, and use your best judgement to decide whether that performance hit is acceptable or not. Always test for performance on your minimum target hardware.
VR refresh rate
Much like traditional monitor refresh rates, this is linked to your game’s frame rate. Consider a 60Hz flat-panel computer monitor. At 60fps, you’ll get the smoothest experience possible because the number of frames that the machine is rendering matches exactly the frequency that the screen can display frames. If your FPS is lower than 60, youll get pauses (delays between frames while the screen waits for the PC) and if its higher than 60 youll get tearing (screen renders the next frame before it’s finished displaying the last frame). At 30fps youll also get a smooth-ish experience because the refresh rate = fps * 2. The same is true in VR, but the effects are way worse. If you drop below your HMD’s refresh rate, youll get ghosting, tracking latency, delays, and basically all the things that make people sick. The current VR standard is 90Hz, so your project needs to hit 90fps or higher 99+% of the time.
No worries, glad to be helping 