Can I guess too?
I think the problem is where the hard line connects to something else. With the pressures many of the efi fuel pumps are putting out, the old rubber hose with a hose clamp can't stand the pressure very long.

I'm running a 97 Dodge efi system on my 39 Dodge truck. Originally I ran a steel line, with slightly flared ends with high pressure efi rubber hose. I clamped the hose to the line with the end of the hose past the flared part of the steel line with a pair of the efi style hose clamps. On 3 different occasions (within a month) The pump blew the hose off the steel lines. Those pumps can pump a lot of gas in a hurry. The only fix that has lasted was when I stepped up to the hard plastic lines with the special ends those plastic line require. Its been well over 6 months and the plastic line are doing well. Haven't had a fuel leak since they were installed.

My guess? The steel lines and short hoses fail too often under the high pressure the efi pumps put out. The flexible plastic lines and their connections do not fail. I would NOT recommend using steel-nickle-copper, or any other hard line with a high pressure hose and high pressure hose clamps. Use the efi designed plastic line with the correct fittings for the plastic lines. Gene