I have a friend who tried to convince me that Beat Saber is "dumbed down" compared to other rhythm games, like Osu. This is false. Let's examine a few rhythm games, including Osu and Beat Saber, and determine which is deepest.
Osu has four "modes", which are essentially completely different games. I'll refer to the one just called "Osu!" as "main".
Played with a mouse or similar device, and has three types of notes: taps, sliders (which require moving the mouse at a steady speed across a path while holding click), and spinners (which require moving the mouse in circles around the center of the screen).
Spinners add basically nothing to the game. Since they always appear on their own (they take up the whole board), they aren't mixed with other types of notes to create variety. Executing a spinner well doesn't require any mental effort, only physical dexterity. And all spinners are the same.
Taps and sliders vary in position (they can be anywhere on the scren) and timing, but this still leaves a pretty simple game: there are only two controls, a clicker/button and a pointer device, and there's no such thing as simultaneous notes.
A scrolling note sheet game that only has two colors or notes mixed into a single track. Each color has two buttons assigned. For most notes you just hit either one. For big notes, you can hit both for more points, but hitting only one still works.
There are two extra types of notes: drumrolls, which contain a rapid sequence of little dots you can hit with any button (and these don't affect your combo), and an obvious spinner equivalent where instead of using a pointer you alternate colors. Both of these are subject to what I said about Osu spinners, leaving Taiko even shaller than Osu main mode because it lacks position. Taiko maps are generally made of extremely repetitive A/B patterns because there's not much else for them to do... except increase the speed so much that it seems physically impossible for a mortal to hit the keys fast enough.
Also, for some reason neither drumrolls nor spinners affect your health or your combo, which makes them feel even more pointless.
A pure piano scroller with no equivalent of spinners, only tap and hold notes. But it's the only mode in the game that can actually have more than one target at once, and it uses a separate key for each button.
Notes fall from the top of the screen and you move a character on the bottom left and right to catch. You have a boost which is necessary to reach some notes.
Catch also features banana swarms, which are similar to spinners in that they always appear on their own, but they are much more interesting than spinners because they require actual thought to find the path that catches the most bananas.
Friday Night Funkin
Almost identical to Osu Mania, except that the number of keys is always four (Mania maps can have more) which reduces the depth, and that unnecessary presses cost health, which I'm not sure if increases depth exactly but I do prefer it, because:
Without it, button mashing sometimes seems better than trying to actually read the notes. I've experienced this a little in Mania and a lot in the many FNF mods that foolishly remove the mechanic.
It adds strategy, since trying to hit a note and missing costs you more health than not trying to hit it. In FNF I've experienced an "acceptable losses" dynamic: I can improve my performance in a hard section by choosing some notes to ignore.
Beat Saber has no equivalent to hold notes. There are only two different colors of notes. While Beat Saber features bombs (which you don't want to cut) and walls (areas you want to avoid with your head), like Osu spinners, these add very little to the game: there are few ways for level designers to use them and most uses are purely decorative because they aren't in the path of notes.
So compared to Osu main, Beat Saber drops hold notes in favor of the color distinction (and thereby the possibility of simultaneous notes). I believe this would be a positive trade on its own, but there's another crucial addition: direction. Beat Saber notes must be hit from a specific direction, which adds a huge dimension of gameplay and level design space that doesn't exist in any Osu mode.
Now, obviously this comparsion is unfair because Beat Saber is a VR game and Osu and FNF are screen games. Also, FNF deserves high praise for not only being free and open source, but working natively on Linux! I think Osu is getting there too, with the Lazer client. But the point stands: Beat Saber is the exact opposite of "dumbed down".
A non-rhythm game: Geometry Dash
Geometry Dash is an interesting study because it only has one control yet it has never felt shallow to me in the way Taiko does. You might say it disproves my assertion that complexity of inputs is directly correlated with depth, but no: the reason Geometry Dash is deep is because you aren't directly told when to press; the depth comes from reading the environment to figure out when to press (this is what makes Geometry Dash a platformer and not a rhythm game). It doesn't apply to most genres, but for rhythm games, there's a very close relationship between complexity of inputs and depth because the rhythm genre is defined by most other sources of depth being absent.