There are a few disadvantages to dual purpose hoppers. The main one is the limitation in capacity compared to rotary gondolas. A typical modern coal hopper has a load capacity of about 236,000 lbs whereas a comparable rotary gondola can carry 244,000 lbs. Although that's only a difference of 8,000 lbs between the cars, on a 125-car unit train that amounts to a difference of 1,000,000 lbs of capacity! Economies of scale still favor high-capacity rotary gondolas in this case. As noted earlier, rotary gondolas are also generally cheaper to purchase and cheaper to maintain.
XCLC 060888 is an example of a typical dual-purpose hopper. It's your generic modern aluminum-bodied automatic-dump hopper equipped with a rotary coupler on one end (indicated by the red end marker).
AMTX 10001 is a dual-purpose hopper equipped with two rotary ends.
CEFX 62321 is an interior-post version of the dual-purpose car type. Here we get a good look at the discharge gates in their open position.