Sickle mowers are made for hay, and pretty much not much else. They're kind of like a disc mower in the sense that you generally don't want to use them outside their intended use as they're not really great tools for handling abuse. Yes they'll knock down some small saplings in your hayfield, but they are not really a repair job you want to get stuck with, and abusing one will quickly result in getting stuck with that repair job. As for a flail mower I grew up using one, but it's probably been 30 years since I have used one. Personally I prefer a bush hog for saplings, but I often will just use our drum mower especially if the blades are dull, and in need of changing anyway to knock down an old field with small saplings starting to sprout up. A drum mower might look like a disc mower, but they are much simpler, and handle abuse much better than a disc mower. Plus a drum mower is a relatively cheap mower, especially for one that works so well for mowing hay.
