Most people won't give a shit about the framerate as long as its playable and fun, I have to remind again that games played on hanhelds are way more forgiving with framerates, as for the multiplayer not being on the box.... doesn't everyone says the multiplayer is shit anyways? For a handheld port Doom looks mindblowing and all previews are pretty positive.
No idea why people are saying that all the time. It has a very old school MP similar to Quake or UT (without the vehicles of course). They did a huge MP update not too long ago and it's quite fun. Also the MP has bots, which isn't too common nowadays.
Good to know then, I just remeber many reviews talking about the multiplayer being quite shitty and people all around mentioning that too, still the game was very praised and liked, but if they improved it that then great, I have no problem to download it, 9GB is a lot but at this point i think is pretty obvious if you get a Switch and you pretend to have many games on it you have to get a sd card sooner or later.