The 8R28 is part of a large family of movements with various complications. The 6R20, 6R21, 6R24 and 6R27 are all in the same family as the 8R28, it's just the 8R28 has a chronograph on the other side as opposed to stuff like a power reserve.
These movements share the basic design of more humble movements like the 7S26 and 6R15, but they are so radically redesigned otherwise that their heritage, outside of how the movement looks, is not obvious. For instance the 6R2X and 8R family is 28800 BPH.
As you can see, the chronograph module is actually on the other side of the movement--all of the movements in this family look almost the same from the back because the complications are hidden behind the mainplate. Also notice the "entry level" Diashock shock absorber, which is a flower on high-end Seiko movements, and the lack of an Etachron fine adjustment screw. These are indications that Seiko probably never developed the movement to be super precise.
As per being against a top-o-the-line ETA, it would normally lose in accuracy since the ETA would be a chronometer. The 8R28 has a much more sophisticated chronograph, however, one of the most advanced in the world right now, and unlike the 7750, is bidirectional winding, which (probably) increases winding efficiency and eliminates rotor noise and rotor wobble.
They did make a more accurate version, called the 8R39, which I've never actually even seen and is rarely used, but apparently has wildly upgraded GS-derived time keeping parts, like the hairspring and escapement.
The accuracy on the 8R28 should be reasonably good, and in ideal conditions, could be very accurate. Where it'll get wonky is when the power reserve is less than half full and when it rests in a position it doesn't like (which can be remedied by figuring out which position balances it out and letting it rest at night in that position). So worn every day, sleeping at night in what you figure out to be the ideal position, it could be quite good.
