The. Martin Baker Mk 4 seat was declared as 0/80, ie safe to use at ground level WITH ZERO descent rate, but 80 knots MINIMUM IAS, and was EXTREMELY reliable, I am pleased to say, having used it in anger.
That said, MB guidance was that if descending then an additional height of at least 10% of ROD was required.
Applying MB guidance, Consider those Snowbird ejections:
It was descending almost vertically at at least 120 K ias, a conservative estimate but maybe even higher.
That is a descent rate of about 12,000 fpm.
That means a minimum ejection height should be about 1200 Feet.
Looking at that video it would appear that ejection was initiated below that height, ie outside the seats capability.
I dont think the seat performed poorly.
On a recent visit to the MB factory they proudly showed me the F35 seat with its auto eject function.
That seat is state of the art and from initiating the ejection to a full chute is extraordinarily fast, I believe less than 2 seconds. The Weber seat is not in that league.