You need to put it in the modern day context with full backs which boils down to not actually requiring as much out of them defensively. Whether it's Trent Alexander-Arnold, Calvin Ramsay or Nathan Patterson, the expectation is that they operate more as wing-backs (or even wingers) than a last line of defence.
I can't say I'm a fan of that approach, but you see it across all the leagues now.
As for poorest fullbacks, the current two wouldn't come close to my top 5 even if you restrict Derek Murray to just one slot.