I'm going to preface this by saying that this is SPECIFICALLY about hypertapping in Tetris. This is not about vibro in rhythm games, as good hypertapping has more criteria to success than vibro does. If you need pure, raw and unadulterated speed, a mechanical keyboard with very light switches is your way to go and I will explain why a bit later. However, there is more than just speed to hypertapping though.
But first let's talk about how D-Pad inputs work on real hardware. On consoles, the D-Pad consists of 3 things: The part of the PCB, the membrane and the plastic button itself. If you press down on one of the D-Pad's 4 sides, it causes the pad to tilt, which in turn also presses down one of the 4 parts of the rubber membrane. Once this connects to the PCB, a circuit closes, which tells the console that one of the 4 directions of the D-Pad has been pressed. It does so at the exact rate at which the console plays back the game (about 60Hz on NTSC), which means you have no added input lag - which can be a saving grace on rare occasions and makes precise timings easier, such as a T-Spin on level 18+.
A rubber dome keyboard works quite similarly. You have a membrane sheet which lays on top of a plate (this can be compared to the PCB of a NES controller in a way), the actual rubberdome sheet (the D-Pad on a NES controller has this too, just circular and for 4 inputs instead of 100+) and the slider with the keycap on it (the plastic D-Pad itself on a NES controller). You press the keycap down, which pushes down a slider. The slider pushes down a rubber dome, which, once it comes in contact with the membrane sheet, closes a circuit, which results in an input. The key difference is the polling rate, which wildly varies between keyboards, but most rubberdomes are at 125-250 or so. However, the polling rate itself generally doesn't have a lot of impact on your gameplay, you might have an unstable input lag variation of about a frame, if at all - the main input lag comes from the emulator itself, if you're wondering.
So, since a rubberdome keyboard and a NES controller create an input event the same way, it also means you can hypertap on them the same way. Since you always know it will actuate when you press down and the key/dpad bottoms out, you get a feel of consistency, so you start using the bottom out as a point of reference and start hypertapping around this point of reference - pushing down for the input, slightly letting go so the input circuit isn't closed anymore, and pushing down again so the NES registers another input. It might sound complicated, but it's a matter of "I push down to bottom out, game does thing. I slightly let go, game stops doing thing, so I press down again so game does thing again".
How is this different from most mechanical keyboards? They work in a completely different manner. I will mainly be talking about MX-Style keyboard switches here, sorry, ALPS users (or other switches that elude my memory right now).
A mechanical keyboard is made of many more parts than a simple rubberdome keyboard is. A single switch consists of a bottom housing, a spring for the weighting, two metal leaves, which, if they touch, cause the input event (as both individual leaves have one leg, and both of those 2 legs are soldered into the keyboard separately, representing one key), a stem, which if pressed down, connects the two leaves I mentioned, and a top housing. On top of the stem goes the keycap, which is what you generally press down on to type. What really happens inside is this: You push down the keycap, and with that, also the stem. Once the stem goes down far enough, the two leaves connect, which closes a circuit and sends an input to the computer. They also can have better polling rates up to 1000Hz, but again, this doesn't really matter, it's just a nice thing to have for other games.
Now, due to the nature of the stem and the leaves, you don't have to press down all the way to generate an input event, which, at first, sounds pretty good, right? Well, there is just one problem: It's really hard to tell where it actuates. Sure, for most MX switches, the event happens at 2mm pushed in, where the stem bottoms out at 4mm or so. Because it's rather hard to tell where the key actuates (yes, even on most tactile switches by Cherry, you can't really feel a whole lot of tactility on them), it's hard to find a reference point. Even if you have the reference point, you can still have a tendency to overshoot or undershoot, because your fingers themselves can't tell whether you triggered the input event, you only have visual/audio feedback with the game doing something. This is why I mentioned vibro at the start of the guide - generally for vibro in rhythm games, you don't need to worry about "overshooting", you just need to press something as fast as you possibly can to pass a chart. Light linear switches are perfect for this. You can use tactiles but they won't be as fast and have a tendency to mess up anyways lol.
With rubberdomes and the D-Pad on the other hand, you know when you pressed down the key, so your finger also knows when it caused the input. This makes it much easier to know when to move your hand back up and down again, resulting in more consistency. It also allows you to more properly count how many times you pressed down, which is very important if you've stacked very high and need a precise hypertap 3 times to the right to get a line piece in the correct position. With mechanical switches you will likely overshoot, unless you are VERY used to them and somehow pinpointed the actual location of the actuation point, which is quite honestly not too likely IMO.
I did mention that light linear switches cause the fastest hypertapping - what does this mean for rubberdomes? They're generally tactile, but if new, can be rather snappy and harder to push down than a mushy membrane, while the mushy membrane might also fail inputs every now and then. Well, here's the kicker: If you can get a membrane to be very light, but still have a good carbon coating on the bottom of the membrane (the carbon is responsible for closing the circuit, and on most mushy membranes, this carbon coating has simply just gone off over time), you will have a god-tier hypertapping experience, as you will have a very light, almost buttery pushdown experience. Because it's very light, you require very little force to press down, which in turn means you can vibrate your arm faster and more "relaxed" - tense hypertapping over time can be much slower than relaxed hypertapping, as you use up less force in what you do, which means you can concentrate on speed instead of force. This is also why light linears are the best for vibro - little force required and you can just go "nyoom" on them. The benefit for a very light rubberdome is having a precise reference point however, which means you get to have the benefits of a light linear while also being able to precisely make out when to stop hypertapping.
Well, this was quite a lot of text and information to digest, so here is a quick TL;DR:
NES D-Pad and Rubberdome keyboards work the same way. Plastic pushes down a rubber dome, which closes a circuit when bottoming out (a carbon layer on the bottom of rubber dome causes this), causing an input event.
Mechanical switches generally work differently.
Rubberdomes and D-Pads are equal, they function the same way. Mechanical keyboards are less precise but can be faster depending on the switch and how used to hypertapping you are on it.
What I'm saying is, unless you assign extra inputs to one action on a keyboard, your performance will be the same on a keyboard as it would be on a controller - that is, if all the parts responsible for inputs work properly.
Hopefully you learned something along the way. If you have questions, let me know on Twitter, Discord, wherever. I'll try my best to answer.