How often can you let the runner access R&D before you must rez ice? Can you take advantage of a rough corp start? Does the opponent learning what you're up to/what cards you're running kill your advantage?
That last one is especially important. I've managed to make a couple of builds that just smash in initial testing against most of the runners, but then gets stomped even harder on the second go-around because my opponent knows what's up.
I do this with goldfishing, but not actual test games. "If I mulligan this, what are my observed chances of drawing better/worse?" is an important question, at least to me. Because I'm crappy and will recall "I don't usually mulligan better than this" better than I will the actual probabilities.
A big advantage of gauntlet testing, I think, is that the players involved can dissect both the new decks but also the current favorites and come to learn how to play (and play against) them so much better. Also, if you do a big huge gauntlet over just a couple of days, you get a much better sense of tournament exhaustion, which is pretty vital to manage. Even back when I played more actively and was probably better, my results tanked come round 4 or 5 because of poor resource management (my body's a resource, right?).
This is also honestly why I think it's good to let the other person play the new deck, too, after you've had a game or three. Maybe it'll be too new and weird and out there, but if you trust their opinion enough they might see things or have suggestions that you wouldn't, normally, and sometimes those'll be worth listening to. Sometimes not, but getting the hands-on experience helps with input when the refining phase comes around.