Seeing as you've compared a GNE StreetLite to the one you've filmed, I assume you were on an Arriva Durham County StreetLite? The maintenance at ADC is appalling, the ones we have North of the Tyne at Northumbria are maintained better, and perform a hell of a lot better than Durham's. Ours can get up a hill at more than 10mph too.
Go North East seem to have theirs set up for power too, whereas Arriva have set theirs up to be more eco-friendly.
You'd be absolutely right, and appalling they are too. They are totally unfit for the 22 and 24 where they spend most of their lives. Meanwhile, 99% of the SB200s have mismatched panels, the Solos smell of soot and the E200s try to rip your eardrums out.
The deckers seem to be generally better than the singles though, the ALX400 B7TLs on the X46 are a pleasure, the E400s are perfectly alright and the DB300s are... well, acceptable is probably the best word.
Best buses in the area are definitely the GNE '53 Scania L94s. Tired as anything but the gearboxes are amazing.