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Large scale AO and indirect shadows may be cast properly only by static geometry.
GI is not working on some forward rendering components like particles or water.
Some artifacts like ghosting, aliasing, light leaking and noise may be noticeable in some cases.
Procedural vegetation and merged vegetation do not cast occlusion or secondary shadows.
If camera was teleported to completely new location it may take up to few seconds until occlusion is working properly.
Only objects and materials with enabled shadow map casting may produce proper bounced light.
For dynamic objects indirect light bounce is working only in the areas near voxelized static geometry.
Bounce light may have noticeable delay of 1-2 frames.
r_Supersampling=2 makes GI look strange but setting lower LowSpecMode (2X lower) pretty much restores the look and speed. Btw temporal AA (r_AntialiasingMode 2/3) works just fine.

Advanced Experimental Options

By default, the GI system released in 3.8.1 is actually only exposing a simplified sub-set of more advanced GI system, which supports features like multiple bounces and ray traced speculars. This advanced system is already in and available, but it is very experimental and is hidden by default. For unlocking advanced settings please enable Edit / Preferences / Experimental Features / Lighting / Total Illumination and then re-open the Environment settings. The description for advanced parameters may be found in the tool tips.

About integration modes

By default in basic mode 0 only opacity is voxelized. This allows very small memory allocations on GPU - about 16 MB. The bounced light is sampled directly from shadow maps (extended to RSM). Compute shaders are not used.

The advantages of mode 0 are:

Small memory usage
Completely dynamic indirect lighting (moving sun does not cause any slowdown)
Dynamic objects can bounce indirect lighting

The disadvantages:

Sometimes low quality of indirect lighting (more noise) especially for small point lights
Only single bounce is possible
Only diffuse GI is possible (environment probes supposed to be used for specular)

Integration modes 1-2 use more memory for voxelization (at least 64 MB) - albedo, normals and several layers of radiance are voxelized together with opacity. The lighting is getting injected into voxelization, then it may be propagated (within the voxelization) and then we sample it during ray tracing pass.

Here you can see example of information stored in voxels: albedo colors, direct light injection and light propagation.

The advantages of this mode:

Support for multiple bounces (light source can be marked to be semi-static with multi-bounce support or to be fully dynamic but only with single bounce)
Support for traced speculars (in mode 2)
Better quality (smoother) indirect lighting

The disadvantages:

More memory usage
Large semi-static multi-bounce lights can not be moved freely (but slowly moving sun may work fine)
Dynamic objects can not affect GI (but can receive it of course)

Tip: If you getting "Display driver stopped responding and has recovered" error message you can use this workaround from Microsoft.