As far as towing is concerned, you can regard the flip much the same as a block, it's just creating a taller ride height.
The problem is (which you've probably read 20+ times because I know I've written this at least that many times) towing a poorly set up trailer, or towing with a vehicle that's not set up to tow. K5's are bit short to be optimum tow rigs but can do it if they're set up right, whether they have a flip kit or not.
I know I'd rather tow with a flip kit than with a block for ride height, and unless you needed the stiffness of a deep arch spring, I wouldn't want a lift spring instead either.
To answer your question specifically, towing in general can be a serious problem, for some vehicles and trailers. I know I've towed trailers with my K5 that were really bad and I could only run about 30 mph without wrecking. I've also towed a fairly heavy 20ft boat up to 70 very comfortably, the major variable in this case was the trailer, the truck didn't change much.
