A well-made butterfly knife (also known as a balisong knife) is the strongest folding knife you can get. The force is distributed to three co-equal load-bearing pins. Keep in mind that in the fleamarket cheapies, what I refer to as the Charming Chineese Cheapies, CCCs for short, those pins are probably made of hollow, soft, aluminum and are set into blow-molded Zamak; even the strongest design is rendered weak if it's made poorly and of poor materials.
The balisong lock mechanism is very binary meaning that it is either locked or not; there's no half-way state. A liner lock, for example -- especially if it's poorly made or even if there's some dirt in it -- can get into a sort of half-way state where the lock portion of the liner moves into position enough to make the blade feel locked, but it is only partially-locked and will collapse under pressure. The danger here is that most people don't look at their liner lock each time they use it to make sure that it locked all the way; if they hear the click and the blade feels secure to finger pressure, they assume it locked fully and is good-to-go. Most -- not all -- other locking mechanism designs have similar partially-locked states that they can get into.
Another major advantage of the balisong design, IMHO, is that it's very simple and very open, very inspectable. You can see the whole lock mechanism and you can inspect it without disassembling the knife. Contrast this with, for example, Benchmade's Axis lock. The Axis lock has two springs in it that both contribute to locking it. There have been cases reported wherein people have disassembled their Axis lock knife and been surprised to find one of the two springs broken. That lock mechanism was half-broken and yet you couldn't tell from outside of the knife because that part of the mechanism is hidden. Since most people don't want to disassemble their knives frequently, it's nice to have a lock mechanism where everything, every part, is observable from outside the knife.
The last nice thing about the balisong lock mechanism is that it doesn't use a spring of any sort. All spring parts will eventually fatigue. And making spring parts is a more complicated and demanding process which means greater defect opportunities.
