78s have a large (relatively) groove which is modulated from side to side. No compression other than that given by the lack of compliance in the cutting head, and by the performer moving towards the mic/horn on quiet passages and vice versa. The only relatively common mechanical reproductions that were modulated vertically were cylinders.
Stereo LPs and singles have the audio modulated on the groove faces at
45 degrees, in such a way that a mono pickup will give a good reproduction of the centre (mono) signal.Because of the groove size, *all* LP styluses will bottom in the groove of a 78, and play a good rendition of scratching the stylus across a sheet of fine sandpaper. 78s must be played with the right size stylus, and for the best quality, you need a selection to account for differing cutting standards and groove wear. Record and stylus wear on 78s isn't helped by the way a lot of cheap 78s had the shellac "stretched" by adding fine slate dust to make it harder and more abrasive, in order to bed in the steel or fibre needle into fitting the groove more quickly at the start.
Another gotcha is that the equalisation varied over time and company by company. However, get a clean early acoustically recorded 78, and it's amazing how good the sound can be, given the right treatment.