I do find using a keyboard for it's original, intended purpose to be quite comfortable, but I can't imagine how I'd live if that was one of my conditions for being able to imagine living.
I don't find keyboard and mouse to be comfortable in any situation for gaming other than sitting at a desk and playing genres that are optimal for it. Laying a board across my lap evokes the feeling of torture racks and unfortunate, lifelong paralysis. The last thing I need to be comfortable in my home, is to be needlessly constrained by things all over and around my body, while I'm trying to relax, and possibly get up to do things. I don't want to feel like I'm in the movie Misery, being served breakfast in bed when I'm trying to chill out and do anything, video games or otherwise.
For genres such as Metal Slug, I find that there's too many superfluous inputs all around your fingers when you're playing a game with digital, cross-directional movement and three other expected actions, as it will be using only a small percentage of the available inputs. This crowded, overabundance of incorrect and unrelated inputs creates a subconscious minefield effect, which constrains your sense of ease and needlessly requires more of your synaptic function (however infinitesimally minute) to consciously consider the actions of multiple fingers performing the same task that a singular thumb could, can and does whilst it slovenly rolls all over a d-pad or face buttons like it's giving it's lover a massage.
I don't even like handhelds with analog sticks too close to the d-pad and buttons, so why would I find this comfortable? I don't even like annoying ergonomic differences from controller to controller, between genres and general comforts, so why would I find keyboards to be infallible and all-encompassing? Trying to play a console or arcade-centric 2D sidescroller with a keyboard is like trying to draw with an Etch A Sketch. Some people do amazing things with those, but I'd prefer to use a pencil and paper.
How is it comfortable? Put it in words so I can understand how it feels to you.
I know I'm kinda hopping in uninvited in this discussion but I found this post interesting and I'd like to say my take on the matter.
I can totally understand the whole lap board thing, it feels like too much effort for something that should have a bit more comfort and allow freedom of movement, and I've never really been able to feel comfortable in such a situation with a KB+M on a couch in any case. That being said, I've grown up playing games at a desk (or at a keyboard I should say) and don't really feel uncomfortable doing that. I like the proximity of the screen and the immediacy and precision a KB+M combination can offer. But I also have a background of playing games on old home computers like the Commodore 64, so using the keyboard wasn't as weird. Also, my dad used to "fix" these old C64 joysticks by resoldering big keyboard keys to the joystick, eliminating the problem of broken joysticks (those things used to break a lot) but also introducing me to the wild concept of more precise, digital input.
Now regarding Metal Slug and other older arcade and console games, I'd just like to preface that I've played these games on original hardware but also a lot on emulators, so playing them on a keyboard is second nature to me.
With all that being said, one could argue that being used to keyboard input for games your whole life (just like being used to controllers) eliminates the discomfort of being overwhelmed by additional surrounding inputs because your hand and fingers always land on the correct keys (I never think about stuff like WASD) and you press the correct keys without having to think about it. Likewise, one could argue that using a separate finger for each action/direction is much more efficient (while that efficiency makes the experience more enjoyable and comfortable), just like actual typing, playing the piano or other musical instruments is much more natural and efficient than using just your thumbs (or any two fingers) to do it, which might even make one feel like they're handicapped, like having amputated fingers etc, not to mention the need to take your thumb off one action to do another might create a very slight feel of anxiety or irritation, because you feel handicapped by not being able to immediately react to any given situation without potentially eliminating the immediate access to another action.
Taking movement out of the equation for a second, I find using the keyboard is pretty much exactly the same as using an arcade stick + buttons combo. I played a ton of arcade games as a kid, and placing the three fingers of my right hand (index, middle, ring) for something like a Capcom fighting game felt very natural to me and it makes pressing the correct punch/kick a very easy, immediate and efficient action. On the other hand, using WASD instead of an arcade stick could also be argued to be even more efficient, because again, you have a separate finger for each direction (except up/down) and can immediately react and move in the opposite direction, instead of moving the entire arm, hand (or thumb) in an arc, from one end to the other. Again, I'm arguing that efficiency can also bring comfort and an overall good feeling to game inputs.
Now of course, you see professional fighting game players use standard console controllers all the time (but also a lot of arcade sticks) and be at the top of the game, but it also proves that you can be just as comfortable and good at it by using an arcade stick, or even something like
Hit Box's stickless arcade controllers which some people seem to prefer and which, if you take into account being used to a keyboard for gaming all your life, feels exactly the same (as in, no confusion or anxiety regarding the other keys on the keyboard).
So I'm just saying that just as you're feeling uncomfortable by having multiple superfluous inputs in the vicinity of your actual main buttons and thinking about more than two fingers for input, you should be able to imagine someone feeling just as uncomfortable by being constrained by just two fingers for input and not having each action (or a small range of actions) assigned to a specific finger, for the aforementioned reasons. So it's not exactly like drawing with an Etch A Sketch, but I can imagine it might feel that way to some people.
There are some obvious ergonomic benefits for using a controller of course, no arguing about that, I'm just saying I find the precision, immediacy and options of the keyboard to overall feel better for me, because I feel I have more control over my actions, ranging from old 2D platformers to FPS/TPS or whatever. I'm kinda clumsy playing Metal Slug on a controller, but I'm very confident and precise when playing with a keyboard.
But then again, I've beaten Super Mario Galaxy in Dolphin with KB+M, plenty of heavy action games like Metal Gear Rising or a couple of DmCs etc. so I'm aware I'm weird that way, and people will undoubtedly cringe at this, but I didn't do that just in spite, or to prove a point to anyone. I play these games on KB+M because it feels so much better to me, like I really have control over my actions and not feeling handicapped by my inputs. That being said, I've also played the Souls series on a controller, partly because the first game did a bad job at KB+M controls, and partly because the game controls really well on a controller anyway, and I have no problem with it.