The only thing bad about a twin circuit panel is the size. I generally go for the straight 40 cir. panels because I hate the mess that there is with a 20-40 panel loaded op. It becomes a rats nest, IMO. Twin breakers are just as safe as regular breakers. In fact I am not sure I every replaced a twin breaker due to failure.
Sometimes the problem is they put the same neutral assembly in the panel as they do for 20 circuit panel, then you may start to run out of neutral terminals if you have 40 circuits - especially if it is also the service panel and you are landing EGC's on same bus.
I don't think much if anything is saved by using twins, and definitely is not with the Quads (two double poles in same unit)
With AFCI protection requirements these days, people that were depending on using twins need to change their ways, but then again, with wall box type AFCI's supposedly coming around -that may change again.
I think you see twins a lot in mobile and manufactured homes for a couple reasons. First they need to pay attention to weight as this home has to be moved down the road at some time. They also probably buy them in bulk quantities and get better pricing than many other people do.