[wrxtasy]

What about S905W Boxes? I have read some complains regarding playback issues and that CE is somehow not as good developed for this Chipset.

Not really, any minor bugs have been ironed out.

Known limitations:
- CPU a bit slower vs all other S9xx chipsets, not much when using CE Kodi. However device runs cooler vs virtually all other S9xx boxes.
- Some deinterlaced OTA TV material can display red artifacts -->>> fixed with a CE Kodi > AMLogic settings toggle.
- 4K (HDR) limited to 30fps = good enough for 24p movies...BUT
- like all S905W/S905X chipset devices there is no Gigabit Ethernet, which is needed for 4K Bluray Rips streaming across a home network.

The ARM G series GPU's in the S905X2/X3, S905D3, and especially the S922X & A311D chipsets are noticeably faster for AMLogic chipset Kodi devices.  

How much noticeably slow when using Android and apps? Usable Android would be a must have beside CE with dual-boot for my purpose. Are there any known best-buy W chipset Box models at the moment, that fits these requirements? Good-priced S905X and S912's are all out of the stock now. 

S905W is too slow for Android IMHO. In fact far to slow. You are talking the lowest specced components used in a AML S9xx device.
It's fine for a lightweight, optimised Linux OS like CoreELEC Kodi.

For cheap dual boot CE / Android you really need to be using a S905X2 / X3 minimum IMHO these days, something with a ARM G-31 series GPU if from cheap China.
And when looking for Android support, find the most popular Freaktab Website S905X2 ROM thread and read.
I would look at the GigaE LAN equipped - 4GB/32GB - X96Max (click)
Careful - usually only 4GB RAM S905X2 devices seem to GigaE LAN.

Mi Box gets away with an older, slightly slower S905X Chipset because they use high quality fast RAM & a great Wireless, responsive remote.

Not all devices are specced equal, or even have the same component quality control.

I'm also would be curious how CE development will advance for VIM3L, are these any restrictions and known bugs yet? And also how usable will be the advertised Android for this Box? Any clue?

No idea.
Should not take much work at all to get CE Kodi on to it. Check out the CE or Khadas forums appropriate thread.

I can tell you the Odroid N2 with it's powerful ARM Bifrost G51 GPU and high quality RAM is nice and snappy running the Lineage based Android TV Pie ROM from high spec eMMC flash storage.
I was quite surprised how smooth and snappy it was for an Open Source Android job.
Pity it does not have Android Widevine L1 DRM or working Android HD audio passthrough.

W.