So, here's the thing.
It's likely that the big brands generally don't make things as good as they could. It's also plausible that the boutique brands take a less compromised approach in many cases.
The problem is that we the consumers have no way to verify the boutique brands' claims. There are tons of anecdotes, but no independent systematic measures. You mainly just have to believe the advertising and testimonials, or not.
At least the big brands carry third-party approvals that can be compared without having to rely on marketing claims alone. For example, when I look at Mobil 1 5w-30, I can tell it places a fair amount of emphasis on fuel economy (ILSAC GF-5) and has really nice high temp deposit control (Honda HTO-06); when I look at Pennzoil Platinum Euro 5w-30, I can tell it's thicker in hot bearings (ACEA A3) and made with long OCIs in mind (BMW LL-01, MB 229.5, etc.).
Every once in a while, there's an avalanche of decent data and feedback suggesting that a boutique oil can address a specific problem with a specific application. One prime example was RLI BioSyn in early direct-injected cars ~10 years ago or so. Some of those cars had hideous fuel dilution issues, and RLI BioSyn (particularly the custom-formulated 5w-40 HD grade) seemed particularly good at dealing with that. In a case like that, there's a solid reason to go for that oil if you have that specific issue. That's the kind of thing that might persuade me to try a boutique brand oil again -- if I had good evidence that it could solve a specific problem that I knew my engine had.
Barring that, it's really tough to say what the benefit of a boutique oil is. By that, I don't mean there isn't a benefit; I literally mean it's hard to know whether there is one.
Most of the time with those brands, there's simply no good way for us customers to tell what's real and what's just marketing.
FWIW, one thing that sets Motul apart from Amsoil and Red Line is that they do make products with those third-party approvals. So there's that.
