I think there are a couple things at play here.
First is driver experience. The Trident in the video looks like it’s being operated at high thruster speed which results in choppy, lunging performance by an inexperienced operator. High speed is better at covering ground in straight lines than tight manuveuring.
It does appear the Gladius may be more stable in pure up/down movement due to more thrusters, I find the Trident likes to pitch up and down slightly, but the apparent benefit ( I do not have a Gladius to compare) is that it can “fly” at higher speed and descend and ascend quicker, and does seem very stable at those speeds. This “flight” inspired design moves differently through the water than traditional ROV’s so it banks in turns like a plane but almost immediately returns to level when going straight or stopping.
I do find the tether can create a significant amount of drag depending on what depth you are operating at, how much line is in the water and boat movement, currents, etc. This instability will probably be similar in all tethered ROV’s with larger, heavier ROV’s more stable than lighter ones.