Horizontal mains runs are the work of the devil bodger.
Indeed, convention exists for a reason.
I'm in the process of removing decades old render from a wall. Beneath lies buried cable, some of it horizontal, some of it vertical. Some of it going to places as yet unknown. There have been buried choc blocks too, all bound up in winds of electrical tape. Call me a coward but I've switched off the power at the main fuse box just to be really
really safe.
Sadly, the thing about convention is it is only that. And even laws/regs get ignored. YOU might know what you're doing, and if you're lucky you might even remember it a couple of years down the track but it can be a hazardous maze for someone coming new to it. If you're going to join cable and change the run direction then it'd be nice/safer to contain it in a junction box with a cover plate. It might not suit the aesthetics but that's another issue!