Those wall surfacing products are usually called for when there is a lot of loss to the integrity of plaster wall surfaces like cracks, crevices or minor separation from the lathe. They are not going to do anything to flatten them out, fix sagging or those kinds of things.
You can skim coat plaster (or even drywall compound to a point) to even out extreme high and low spots. Otherwise chalk the flatness issue up to the vintage of the house and think of it as character building---at least until you can replaster or drywall. Plaster walls and ceilings were never as flat as drywall. It was a real credit to craftsmanship and skill of the paster contractor to get them as close as many are. Not many around who do it anymore.
You can skim coat plaster (or even drywall compound to a point) to even out extreme high and low spots. Otherwise chalk the flatness issue up to the vintage of the house and think of it as character building---at least until you can replaster or drywall. Plaster walls and ceilings were never as flat as drywall. It was a real credit to craftsmanship and skill of the paster contractor to get them as close as many are. Not many around who do it anymore.