Issue with the tripod ladder is that it can fall pretty easily to one side or the other. If the 3rd leg is not perfectly parallel to the other 2 legs and / or not perfectly perpendicular to the ground and / or not equally spaced between the other 2 legs. In this circumstance, a subtle weight shift to one side or the other causes the 3rd leg to cave in, since it's very weakly attached to the top step. On solid soil ground, it might be OK. On soft sand or mulchy ground, it's easy to create a tip over condition. This is much harder to do with a 4 legged ladder or even a 2 legged extension ladder.
I've owned all 3: tripod, 4 legged, and extension. My take is: tripod or 4 legged is best for heights up to about 10 feet. Extension ladder is best for anything beyond that (eg, my 30+ foot canistel).