Virtual Texturing, UDIMs and Lightmaps

Any Virtual Texturing experts?
The topic is somewhat confusing because of the many options.
There is RVT (Runtime Virtual texturing) and SVT (Streaming Virtual Texturing) and also the option to enable virtual textures for your Lightmaps.

2 questions.
If I want to make use of UDIMs (more than a single UV and texture atlas for a material) does that mean I must use SVT?

If the following is true
“Enabling support for virtual texture lightmaps can improve streaming performance and quality of lightmap bakes.”
why wouldn’t you always leave it enabled in your Project settings?
ie what are the downsides to virtual texturing and lightmaps?

Thanks!