I personally use a blade fin, but would not consider split fins to be “dangerous.” As with all dive gear, there are pros and cons you should consider based on the type of diving you plan to do.
The idea behind split fins is that instead of just propelling the diver forward, the split creates a vortex that assists in propulsion and speed. Another benefit of splits is that the opening allows water to pass through easily on the upward fin stroke. Since the upward fin stroke is the movement that achieves the least amount of propulsion, having the split is more energy efficient than having to kick upward with a standard blade fins. This makes the split fins easier to use in many cases and allows divers to conserve air because they don’t have to kick as hard.
As a result, split fins are good for casual divers using a scissor kick, inexperienced divers who may not have good kicking technique yet, and divers who have ankle or knee problems and cramp easily. However, blade fins are more powerful, and many divers argue that split fins do not provide enough power to fight through tough currents.
So, split fins are more efficient in terms of propulsion and oxygen conservation but blade fins provide more powerful thrusts. The type of fin used should depend on the individual and the type of diving they plan to do. If you aren’t sure what’s right for you, stop by your local dive shop and talk to them - they will be able to help you make the right choice for your diving plans.