They may be stronger at the corners, but the thin tubes can crack easily a la the C50 from Colnago (in a crash, but that’s just as with any carbon tubed bike, lugged or not). I have not seen any particular evidence of stress cracks appearing more frequentlyat the edges of the lugs, but that’s where they would appear on a straight gauge tube. But then, some of the older Colnago Carbitubos are still going, and that’s a lugged frame. Colnago only does lugged carbon frames, I think, but I believe that a monocoque (properly designed and laid up) has the potential to be far stronger.
It is difficult to make generalisations, though, as there is far more variability in lay up techniques and the design skills of different manufacturers than with steel frames. Another problem is that you can only see the outermost layer of carbon, and you have no idea of what goes on underneath.
But Maunaloa does have a point that a CF frame will be more likely to collapse before a steel frame, or a Ti one, because they will eventually delaminate or corrode, at places like water bottle bosses and where perforations are made through the tube, like brake bosses, or the seat tube. Not within a year, and not all at the same rate, of course. Steer clear of magnesium frames for they have terrible corrosion problems (even with chrome overplating). CF does actually contribute to electrolytic corrosion against aluminum or steel if the outer plastic layer is rubbed off, as the carbon fibres are conductive.