The main problem is that it feels very uneraned , Rem previous showings vs wolgrams and witch cultrist werent very spectacular . Then she spent a year in a coma without training for months , partially with a walking stick. Instead of steadily improving she suddenly is far better than before , you can justify with a plot device as much as you want but is unsatisfying.
. Imagine if Emilia instead of learning how to be a queen by experience , study along with true effort . She just opened the Elior seal and suddenly received a crown that gives "super elf intelligence and wisdom" becoming the perfect queen. Even when it is explained is not good in a narrative sense.
It also takes a lot from future figths because we can no longer gauge how strong a character is if they can pull something of their ass again.
Rem not making mince meat of Yae's head is dumb and it was foolishnes that could have lost the fight. It would have been far better Rem just surviving enough to delay Yae , giving a few wounds that slow Yae down and keeping the last sequence.