We scraped our entire house. What we did:
We did try the just-a-sander approach. Eventually you'll mess up and push too hard, and create a bare drywall spot, making the surface completely uneven.
The process wasn't difficult, just really time consuming. We even had vaulted ceilings that required a really tall ladder. While going through this you think it's not worth the effort, but then you see the finished product, and are glad you did it.
Buy this:
Use 120 grit sandpaper. Wear a good dust mask and seal off the room with plastic sheeting.
Before you begin your process, buy one of these and sand your ceilings. It will decrease your work by 50% or more.
I had 'stomp' patterns on my ceiling, I sanded it and found the texture to be very pleasant, so it was primered and painted. I get compliments on the outcome.
Before I remodeled my kitchen, I bought one of these. My walls were so bad I had to skim everything. It was cheap enough and was willing to risk the cost if it was at all convenient and it was. Very, very glad I got it but it's not without its own issues.
Connected to a powerful shopvac, it's amazing how much dust it removes - but there's a learning curve on how long to let it hover and when working ceilings, it gets heavy and that's when I started contorting into random shapes that caused some divots. You'll still need to use a sponge and pole sander for the corners, but I'm glad I have it.
One of these will help speed things along and somewhat reduce the dust. Can be a bit of an upper body workout but for large areas of drywall, well worth it.
WEN 6369 Variable Speed 5 Amp Drywall Sander with 15' Hose https://smile.amazon.com/dp/B01HRL9XYI
Here's the one I use. Nothing fancy, but its been great so far.
If you have a lot of sanding in your future, especially large wall sections, do yourself a favor and get one of these. Pays for itself in just a couple of hours - so SO handy. https://www.amazon.com/dp/B01HRL9XYI/ref=cm_sw_r_cp_apap_U6mg92vFi2sQn
You want to smooth plaster or drywall?:
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https://www.amazon.com/WEN-6369-Variable-Drywall-Sander/dp/B01HRL9XYI
Yep and easy-ish? So the bit where you reattach any broken keys to the lathe via washers is bonkers easy (I taught my Mom how to do it last weekend, here's a this old house tutorial for your perusal https://www.youtube.com/watch?v=P4D0sESi5So)
The hard bit is learning how to use joint compound in such a way so you don't have to sand for 19 hours. If the walls are already in fairly good shape then it's not hard to just fill in the holes with joint compound and do some light sanding but if the walls are shot to hell then you need to learn how to skim coat which is a form of hell on earth. The walls in this room aren't terrible barring that one really shitty corner so we are just patching up the worst bits and wet sanding which is pretty easy and also doesn't cover your entire house in drywall dust, but I'm also good enough now that I can do light patching like that and feather it out to near flat without sanding so it's not taking us long to sand. This guy has a good video on wet sanding (https://www.youtube.com/watch?v=BTLFZbSZuPY).
However we did another room in our house which had the plaster in WAY worse shape (people that paint over drywall have a special spot in hell) and so had to skim coat the whole damn thing + sand it all down which was zero fun and took forever (though the results are gorgeous). If you really want to get into that nightmarish mess I recommend this technique (https://www.youtube.com/watch?v=N2RSDsYbA7Y) which assumes you have no idea how to use a hawk and trowel. Also just buy/rent an electric wall sander, you are not going to be good enough to get the whole thing flat even with that method and trying to do it via those pole sanders is awful and you will put a big gouge in your wall. I was actually pleased by how well this sander worked (https://www.amazon.com/gp/product/B01HRL9XYI/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1) despite the pretty cheap price point. Probably not great if you are doing this for a living but for a room or two over time it actually worked pretty well.
What I'd actually recommend if you get to the point of needing a whole skim coat is do the plaster repair bit yourself then just hire out the skim coating. Drywall guys can do a skim coat in like a day (vs the week it will take you), it's bizarrely cheap for what they do and they are much better at it then you ever thought about being.
Also once you are done with all this repair Killz Adhesion is an amazing primer for it. Single coat of it over fresh mud made it so we could do a single coat of high quality paint over it and it didn't soak up all the paint nearly instantly.