is that they are not able to make a reliable, consistent, AF, on their lenses for Canon.
And yet I've been using my Siggy 120-300mm f/2.8 OS hard for nigh on three years, shooting birds, aircraft, fast bikes, moto-x. touring cars - all the stuff that stresses an AF system - and the AF has not missed a beat.
So that's some first-hand experience for you, right there: apparently they can "make a reliable, consistent, AF, on their lenses for Canon".
Same story - over a shorter period (but still over a year of hard use) - for my 70-200mm f/2.8 OS.
Siggy users aren't idiots, nor or they inherently easily pleased: we use the lenses because they're bloody good at everything they need to be good at.
For sure. I've used a Sig 70-200 f2.8 HSM for tons of basketball and hockey with zero AF problems and my new (to me) 300-800 is tack sharp with no AFMA required. It also hasn't missed a shot due to AF yet. All of this AF reliability talk is overblown FUD passed around by the same handful of sources on camera forums and review sites, Bryan from TDP being the worst. His reviews and lens comparison tools are usually great but that guy is either the unluckiest camera enthusiast on the net or he has an axe to grind because every review he does of a Sigma lens starts off with him talking about having to return half a dozen copies before finding one that works and that no matter what the AF is always bad, which flies in the face of every other review on the net, my own experience and that of IRL friends of mine.
There's probably also a lot of confirmation bias out there, too. People have been told that Sigma AF is unreliable so they are more aware of it when it fails and immediately chalk it up to poor engineering while ignoring similar failures in first party glass. As an anecdotal example, I was using my Canon 300mm f2.8 to shoot tight portraits at a basketball game and the focus would constantly miss the eyes, inconsistently front and back focusing. This wasn't a problem with the lens; the conditions were just very difficult - thin DOF, low light, moving subject. No one would fault poor engineering for the AF failing because it is Canon but if that were a Sigma lens in the same situation you can bet the blame would fall on the manufacturer. There are plenty of Canon (and Nikon) lenses that have reputations for slow/inconsistent AF but it doesn't tarnish the whole company's product line because people are willing to be more reasonable with a certain amount of failures. I mean, hell, the 1DIII, their flagship pro camera body, was a spectacular
failure on the AF front and yet Canon survived that debacle pretty much unscathed.
Sigma has been making some fantastic lenses in the last several years. I think that makes some people upset and they would really like for there to be some major downside to buying a third party lens, especially when some of them are optically superior to whats available from the OEM.