OBA, Transit, Apple Maps, and King County Trip Planner all use the same back end data sources (though Transit adds users who are volunteering their location with the Go feature in its app) so they should all be similar to each other.

Ever since the major overhaul of data in mid-2015 that tried to fix the "ghost bus" problem (a bus would show as being on the wrong trip pair) and the "never-ending terminal" problem (arrival times for stops very near a route's terminal would show as many minutes delayed or early until the bus actually started moving), I've been quite pleased with the quality of real-time data.

There are always going to be inaccuracies, though. At peak, Sound Transit / OneBusAway are trying to track a couple thousand vehicles across many miles in unpredictable traffic and weather. That it works at all is a minor miracle in my eyes.