Yes—you can focus with the ND filter on if your camera can do it reliably.

The main drawback is simply reduced light. Any focusing method—optical viewfinder AF, contrast-detect AF in live view, or manual focus by eye—has less information to work with through a 10-stop filter. Live view often works better because it uses the main sensor and can amplify the signal, but it may become slower or less reliable in very low light, and noise can affect autofocus accuracy.

So in practice: if live view AF locks accurately for your scene, there’s no real problem using it. For tripod-based long exposures, the slower speed of live view AF usually isn’t an issue.

A common alternative is to focus first, then switch to manual focus before attaching the filter. That avoids AF problems, but be careful not to shift focus while screwing the filter on.

For maximum precision, use live view with manual focus and magnify the image to check sharpness. You can also pre-focus by estimating distance and using the lens distance scale when available.

Metering may also become less reliable in very low light, so checking exposure and adjusting as needed is normal.