Well, it’s semantics. Aluminum is cheaper than magnesium. Therefore, it’s also less expensive. But when there’s a large disparity in cost, there’s a reason for it.
...the 60-600 is built with magnesium.
If you want to argue that there are trade offs, I would agree. Even within Canon's own lenses. There are always trade offs.
This lens will be pretty cheap compared to its rivals. How good is it expected to be? And are cheaper materials causing the extra weight?
But your original question was how good is it expected to be.
I would expect it to be very good.
There are some that have had AF problems. There are others that haven't. But build quality is not lacking. Optical quality is not lacking (some do not like the color or bokeh, both are subjective). But a lot of people are using Sigma glass and loving it.
As for the weight, for the 150-600S you could argue that the aluminum came into play. My take has been the glass. Canon uses a number of different optical elements, such as fluorite, to minimize weight. I also think they have worked with the optical design to minimize weight, just look at the Mk III of the EF 400 f/2.8 and EF 600 f/4. I suspect Sigma optimizes their design for optical excellence and cost.
Trade offs. But, for a 10x zoom range, good chance this is a great lens.