Every cracked Flues that I have seen has been a 20. Sure that there are others out there, that I have not seen, but the 8-10 that I have seen have all been in 20 only.
It has been suggested here that the width of the 20 along with the thickness could be an indicator of possible light weight construction and therefore potential problems.
I rather think that the real problem is modern loads in a gun not design for the high pressures. If your gun is designed to handle 8-9,000psi and you feed it 10-12,000psi be happy the water tables fails before the barrels burst. A crack is a bummer but a ruptured barrel can ruin you future, your future shooting and possible the number you can count to on your fingers.
The caution about the thin 20 barrels is about not having the chambers cut to 2 3/4" as the amount of metal in the chamber areas can be marginal when done. This just encourages shooting high pressure shells, with now thinner barrels and only bad thing happen if a barrel fails in the chamber area.
