Gear cables shearing off inside levers

I've got a Merida cyclocross bike with 105 5800 and internal cable routing. Never been happy with the shifting: very stiff compared with previous bike with much older 105 and external cable routing. A couple of months ago, the rear shifting went to shit - jumping around between cogs, needing several lever-presses before it would respond, not getting to the bigger cogs- before it snapped and got stuck on the smallest cog. Went to LBS - cable sheared off inside the lever as predicted. Got it sorted, but shifting still shift, and now it feels like it's going again.

Is it a common problem with 5800 shifters? Could the tight bends for the internal routing be to blame? Better cables needed? A compound problem? Don't want to throw money at the issue until I know what it is...

All shifters knack cables.

common shimano problem

you need to catch it early, so when you click to shift and it doesn't respond is often indicative that one strand of cable has frayed inside the shifter. continue using it and the frayed strand frays more of the cable and then hey presto, click to shift and it snaps

next is about an hour of fannying around trying to pull the cable out

Seems to be a problem with Shimano 11. Have never had this problem with older Shimano, SRAM or Campagnolo.

had same problem with DA 7800

Never had 7800, although never had the problem with 6600 which I thought would be pretty much the same, perhaps not.

Didn't have a single cable go like this in seven years of 5600 ownership. Have not liked
these levers from the get-go; feel way more industrial than previous IMO.

Could drag from the cable routing be contributing or is that blind alley?

I had this happen over and over again on my 10 speed 105. I think I've had it once in about 5000mi on my 6800 11 speed ultegra.

As mentioned above, spotting it early is key. As soon as shifting starts feeling dodgy and can't be sorted with a tweak of the inline adjuster or whatever, it's probably the cable going.

Will powerfully expensive Yokozuna cables help?

I think in most shifters the cable has to make a 90 degree turn in two axes (sideways then vertical then back) inside the shifter compared to the brake cable which is pretty much a straight line through front to back

No, as mentioned it is how the cable wraps inside the shifter that causes the problem.