Like a few other people in this thread I tried roku, firetv box/stick and landed on an nvidia shield to get away from ads. At the time of writing (now that shield/androidtv are showing ads) I don't believe a decent ad-free box exists.
I don't have one or use OSMC (although it looks interesting, not sure how i missed it).
I currently use an Raspberry Pi 4 with Raspbian and have Kodi installed for the media centre experience and use Chromium with a "hack" which copies the ARM widevine plug-in from Chrome OS so DRM streaming works.
You can just use Netflix, BBC, whatever through their website in most cases. Streaming is fine for me at 720p, and i think 1080p also works well enough. More than 1080 and an RPi 4 doesn't have the decoding power right now i think.
It is a clunky setup though, OSMC and their hardware is probably much slicker, but you would loose the ability to watch streaming services (that require DRM at least).
That said: I thought I would be fine using it for local content and the TV for streaming content however over time having to use different devices for different content is definitely starting to get annoying, and I'm considering picking up an Apple TV because of this.
I use it just fine. Not sure what adverts you are referring to past the tv season suggestion on the home screen, but I use a pihole.
You can use a recent Pi if you want plain Linux. It evens works with CEC. No idea about YouTube though (I share the video to the Fire TV Stick and it plays).
It also requires signing in at all times (not 100% sure on this) to use the device so I don't consider it a viable alternative to a "dumb" android box which has a plain experience throughout