Nitrate levels in unprocessed beet produce is highly variable (beet varieties, agronomic practices … especially fertilizer use, timing of harvest, weather conditions). This can range from almost none to quite a lot. To have a positive effect you actually need quite a lot nitrate.
If you’re really after the benefits for performance there is no alternative to a standardized product. Wether unprocessed beets are healthier or not is a totally different story.
With regards to juice, no, nitrate is quite stable. However, as alluded to before, the bigger uncertainty is natural variability of the content.
What I find sort of funny, I have a background in “molecular quality of vegetables” … let’s put it this way … at uni I learned how bad nitrate is zillions of $ were spent on research projects to reduce the content. And now we seek maximum content to ride a bike faster … and perhaps all the fuss then was just a fuss. Like most things diet related …