ALL subsequent attacks seem to have the reduced power unless you block-cancel.
e.g a Mk 5 Combate Axe has a heavy attack chain (combo) of 2 overhead swings.
With LimbSplitter the 1st heavy overhead will be buffed, the 2nd reduced, but then the next one (3rd in a row) which should be a the first attack in a new chain and all others remain reduced in damage.
It technically doesn’t say “subsequent attacks in the chain” have reduced power. But you are correct, the blessing is a complete and total nerf to the weapon that gets it. Some very few people use it on some very specific builds where they block cancel every single attack, but yeah, it’s a blessing that can completely ruin a weapon.
Unfortunately it seems to be a very common blessing, yeah.
Even more unfortunately blessings don’t have an equal chance of appearing on a weapon. The really bad blessings have a much higher chance of appearing (through store or when upgrading).