I saw about three things wrong with that boat
1) Balsa framing
2) Not built on a jig, could be twisted
3) The way he installed the flotation

For a high performance boat, balsa isn't a good choise of materials. It acts like a sponge around water, absorbing water almost as fast as it gets into the boat. It's also not strong enough to take the pounding a boat takes at high speed, so leave the balsa for the airplane builders. I would recommend aircraft grade plywood, using 1/16th for the sponson insides, the bottom and the sponson bottoms, For all the rest of the framing and skin, 1/32nd ply, while the sponson transoms and stern transom should be 1/8th ply for strength. I would also use 1/8 spruce square stock for glue blocks to reenforce all the 90 degree joints

A boat of this type needs to be built on a jig to prevent having a twisted hull. The builder may have a twist in the hull and not even know it, causing some of his problems.

The third thing was the flotation. He had all the foam in the back half of the boat, This tends to put extra weight on the rear, ultimately making the boat more prone to blow overs, as demonstrated in the pics. On a boat of this type, you want the boat to have less than 10% of the weight at the back of the boat when balanced on the sponson transoms. You have the weight of the rudder assembly, the strut, prop and driveline, as well as the back half of the hull to begin with. By adding all that foam, the boat is stern heavy. He will need to rebalance the boat to fix his blow over problem, if he can fix it at all.

Having said what I didn't like, I will say what I do like. I like the design. It's simple, easy to build, and with the sponson bottoms angled up at 6 degrees from the sponson transoms to the bow, looks like it could be a fun little boat. If you like the design, get a set of plans. By following them and using the proper materials, it could be a fun project