I use Shield Pro 2019 since Nov 2019, have been keeping up on software updates -- haven't ever noticed ads. I use Pi-hole though, maybe that has been 'shielding' me (ha!) all this time and I didn't realize it?
(To clarify I mean no ads in the Shield UI itself, I'd still see ads in individual streaming apps like Hulu)
For me the default launcher would always first list "recommendations" to install Disney plus or other platforms, before showing content that I'm actually interested in.
I have since switched to a different launcher, one that automatically starts up plex as well, since that's all I use it for anyway.
I just got Projectivy Launcher. You need to get premium (one-time 7.25 cost) to set it as the default but it has some nice customizations. Works great on my Shield and a Google TV.