To quote Clint Eastwood: "Do you feel lucky?"
The rules are the rules. Will they be enforced? Will they be strictly enforced? See Clint, above.
If you want to be 100% sure there will be no surprises, you follow the rules, to a Tee. If you are willing to roll the dice, be ready for whatever consequences come.
I have flown Lufthansa twice in the past year, including 8 days ago. Our carry-on luggage would have been able to sqeeeeeeze into their metal box, but only with with some firm pressure applied. I think for many folks, it's the weight limit that is the challenge. Mine would have been just barely over the weight limit, but probably within the margin of error for their scale/my scale, and I gambled that it would have passed muster for both size and weight if they checked, though I might have gotten some stinkeye.
In both cases, nobody was checking size or weight, and we skated right through like on our permissive, anything-goes domestic airlines. When I flew from Bergen to Frankfurt on Lufthansa 8 days ago, I took a good look around our gate and found no bag sizers that I could see, and gate agents seemed completely disinterested in scrutinizing bags - I saw some big rolling carryons that looked way, way over their limits, making mine look very compliant by comparison.
But it's all up to the hands of fate. Ask Clint about that.