Are you looking at older dates? if you look at the report in Site Traffic for a date that predates when it was originally rolled out, the data will show as 0
From the commit that added the report:
All pageview data in the new report before the date where the first
anon or logged in browser pageview was recorded is now hidden.
I believe this is because the reports don’t line up 1:1, so this was to avoid inaccurate comparisons between the two. So yes, going forward you can ignore the legacy report.