On the other hand
- they are not that old
- Sony tried very hard to make them the best such lenses on the market to convince DSLR shooters to change over
- Sony had plenty of experience designing such lenses
- they were clearly priced as if they were the best
So until they release a competitive high end zoom we don’t know if the constraints of the mount enable them to be at the same level as Canon/Nikon.
They are some great primes for FE mount though and my many Sony friends have all become prime shooters.
Cheers,
Bernard
Given the pace of mirrorless development since 2013, they're pretty old. Also, they were designed with a different intent to current design. Originally, the GM series was designed with bokeh rendering in mind, not necessarily ultimate resolution. And the 24-70 GM and 70-200 GM definitely have among the smoothest out-of-focus rendering out of lenses in their class, so they certainly fulfilled their design intent at the time. But the design intent of GM lenses has changed over the years.
Compare and contrast the 100-400 GM. Unlike the original batch of GM lenses, resolution was a design intent in this lens. Matches the Canon in centre sharpness and beats it in the corners. Also handily beats the 70-200 GM at the same aperture/focal length. Both the Sony and Canon beat anything Nikon has in that class.
And, looking at the new 12-24/2.8 GM, they've now done the same thing here - still waiting on formal tests, but it seems to beat anything else in that class (Canon 11-24L, Nikon 14-24, Sigma 12-24 and 14-24).
The mount is obviously not a constraining factor - not unless you want to get in to f/0.8 lenses or something. After all, the Sigma 105/1.4 works perfectly well (optically) on E-mount, as does the designed-for-E-mount Sigma 35/1.2.
You can't judge a system by its first few products - only its mature offerings. The Canon R is crap. They've now refined it with a second generation. The A7r was useless for anything other than landscapes or still studio setups. The A7r2 improved on this, and, from the A9 and A7r3 onwards, the system has really matured. Same goes with the lenses. Look at what Sigma used to produce before the Art and Sport lines came out.