My 'problem' with the Theory is the same as it always has been, and it is "my problem," not the discs. Even though I'm much improved even since I got the first Theory I got, it still falls in a weird way with my "margin of error," even though that margin is much smaller now than then.
Slight miss on the hit, or pulling through a bit too wide, etc, and I get straight, glideless flight, or on taller shots, stall and fade instead of long and straight like a 9 iron. Perfect form, I get workable US mid lines. That's just not useful for tight-lines golf, for me, to not be 100% sure which I'll be getting.
The Relay, at that same distance, is more predictable for me, by far. Plus it can do longer lines too. Plus it's better for control/bad lie flicks.
The Theory is still good at a few things so it's in the bag. It'll hold a high, steep, powered down anny like a champ, for ex, better than a Proxy, even.