I've had trouble with cracked line coating just above braided loops and nailknotted leader butts. At least part of the fault was mine: I used stiff and too-thick finish over these parts. Something like Flexament or Pliobond probably flexes enough to transmit energy more smoothly. And some brands of line seem to have less durable coating than others; but my experience isn't comprehensive, so I won't libel anyone's product here.
Speyman is on to an important point. For a long time, the advice in print has been to use heavy, heavier diameters for leader butts. I observed early in my experience that conventional wisdom may be unrealistic. Watching a friend cast my rod, I could easily see that the smoothly unfolding loop of my 444 WF7F was suddenly interrupted by the resistance of the .025" Maxima butt. I experimented, and found that a butt as small as .013" in a 12-foot leader would turn over slowly but properly with small flies.
There's a simple, empirical test I've used for decades to match fly line tips to proper leader butts: Take four or five inches of the line tip between finger and thumbs on each hand; flex it a few times to get a feel for its resistance. Now pick leader butt material that feels the same when tested the same way. Use it. It will work.