That's a very low efficiency mob grinder, and it contains a specific flaw that makes it even worse.
1) The mobs need to move on their own, entering the flow. That means you need to be a specific distance from the spawn area for it to work at all. Covered here:
What is the optimum player position respective to mob grinder?
2) mobs are not suicidal - they fight against the water current and try to exit the "flow" if they can. Only once they are so crowded they push each other, they start falling. That used to be non-issue if you obscured the hole with sign-posts, but that is no longer the case. You need some tricks like pistons or redstone-activated trapdoors now to get the mobs to drop without being pushed down.
3) This is built on ground level. There's probably an extensive cavern system below, meaning a lot of mobs spawn there instead of in your grinder. Make sure all caverns within 128m are lit - or move your grinder waaay up.
4) The author of that video is reaaaally asking for trouble. First, light the area under the grinder. Even evening/rain light levels will let mobs spawn there. Then build the bottom section of the drop chute (flooring + at least one ring of the chute) out of obsidian. Some creepers WILL blow up, and then you'll have an unrestricted rain of mobs dropping while you try to repair the grinder. Also, watch out for baby zombies and chicken jockeys. Wear an armor.