Oh boy this one is going to develop nicely...
Well since it's going to shit, here's some opinions of mine... Take them for what they're worth.
AI is an awesome system, action, etc... The AT, the AX, AW's, AE, whatever. All top notch. However, there are some drawbacks. If you don't like the AI chassis, or viper skins, you're kind of in the shit house. They do have issues with some calibers and primer combinations that pierce primers, and although the trigger is great, very reliable, etc... If you don't like it for whatever reason, you're in the shit house. You can't make everyone happy all at once.
With customs, really I look at the Mausingfield and Bighorn TL3 (The Nucleus looks nice, but I don't like the sandwich lug, and it hasn't even hit the shelves yet). CRF and manual ejection is my thing, and I especially like the M5's Mauser style extractor. Here you're at the mercy of whoever is building it. You can run switch-barrels but it requires tools or a Barloc/switchlug, and IMO they're never going to be on the same level of ease of use as an AI, but I guess it's worth mentioning. The other benefit here is that you can pick a stock and a trigger that fit you exactly, or have them made to fit you exactly. Chassis/minichassis options and prefit barrels are out there now that assembling totally stock, high quality parts by the end user is, I think, approaching the level of AI.
Consider the following,
Manners T4a(or any other T#a) mini chassis, or whatever quality chassis you prefer...
M5 Mausingfield
M4 Barloc
PVA Prefit shouldered barrel
Trigger Tech trigger
ARC Magazines
You're in the $3500-4000 range, right in there with an AI AT. Same accuracy potential, very robust/reliable system, and potentially more versatility in stock fit/feel, barrel length/caliber/profile, overall weight, the ability to do SAUM/WSM, etc.. AND These are all to-spec, not fucked with, no custom fitting required high quality parts.
Not going to say it's the end-all be-all for everyone, but neither is the AI....
So to the OP's question, I maintain that a Defiance isn't on the same level as an AI. AI wins hands down if you're ever going to run it hard in the dirt. On the slight side-bar of is an AI better than every custom build ever? I think that's entirely dependent on the end user because IMO you can put together some stuff these days that is pretty effin bomb proof.