Are you saying that it "locks" too early, and, because of this, even under / after heavy loading, the last part of the Tail End remains slack - meaning that the last tuck through the collar-around-the-Standing-End is more or less redundant ? That would be a disadvantage, indeed. In an "ideal" practical knot, we would had wished that each and every part of the whole would contribute to its security and strength to an equivalent degree - that is, we would had wished that the tensile forces would be dissipated / absorbed by the whole nub. Of course, this can not be achieved 100% in any real knot - but that does not mean we should be satisfied by not-compact knots, with parts which remain slack and "decorative" !
I have also expressed the opinion that
the last line of defence against slippage, the one that blocks the movement of the last part of the Tail, should be utilized more than the other parts, because, in this way, the loaded knot would be always self-dressed in a compact form : such a Standing Part, where no segment would be allowed to remain much less tensioned or even slack, will always force the whole nub to "fold" in a compact form.
With soft ropes, it is true that it is always more difficult to
push a particular segment, and feed more rope length into a "locked" area of the nub, in order to release the tension. If there is no available "handle" to grasp a bight and be able to
pull some rope length out of this area, untying the knot would be more difficult, indeed. I have never tied and loaded so heavily as you did this knot on a soft rope, so I do not have a "feeling" of the difficulty you describe, but I think I can understand it.
What can I say ? You tie a simple knot, and you worry because the Tail can slip. You tie a just one tuck more complex one, and you worry because now the same Tail does not slip at all, and remains slack !
Catch 22 !
