We have a ton of older houses over here in NZ, that still have the lath and plaster walls and ceilings.
Over the years, I've tried various methods to cut that stuff, with widely varying results.
I think the key to cutting this stuff, without tears, is to use a tool that will cut quickly with the minimum of vibration to the surrounding laths, that is what will cause things to turn to custard in a real hurry.
I have used a Dremel tool with a cutting disc in it before today to cut out "flush-box" holes in walls, for new sockets and light switches.
I have also used, at a pinch, a 4" grinder with a cut-off disc in it too, just make sure you ascertain where any timber joists are behind the laths, before you start cutting, marking your final cut in pencil gives you a good guide.
One thing needs to be said here, if you are going to be using power tools like this to do the cutting, you'd better have very good control of said tool, otherwise you could find it sailing across the wall or ceiling all of a sudden.
Not a good look.
One bit of advice I got from a guy I served my time with and have never forgotten is, if you have to make a hole in anything, always start small, filler is cheap, this also allows you to "move" your hole around to a certain degree, if it is slightly in the wrong place, making a big hole for a start, helps no-one and costs us money and time to get it fixed.