The Streetdecks were sadly not up to the task of running the 376 with its mix of city running, high speed country running and a few long climbs out of Pensford/Farrington or Bristol Hill (and some other still taxing though less severe such as out of Chewton Mendip). Seems that the Streetdeck just doesn't like that mix and is better suited to town work. The e400s are fine for the routes at least for a couple of years. ADL e400s manage everywhere else in the country with fleets of them running for First, Stagecoach and Arriva though perhaps the odd one will be a little temperamental after a few years.
It's the same with the B9s - generally good though a couple of those have a bit of downtime since they headed to Wells but in a couple of years, they will be like 32669 et al and hanging on being 14 years old. As for the ex Leeds B9s, I've been on a couple (can't recall which ones) but they seemed quite sluggish. Still, reading all the Leeds area adverts inside (6 months after transfer) took my mind off it. It's notable that they've been happy to send ex Leeds ones to Wells but not use them on the X39 where the native ones and the ex Bolton examples were usually there prior to the Scanias.
I would make a caveat about the 14 plate e400s and it's not simply because they've been on city work so they can't cope with fresh air. You've had vehicles that have been on loan to Diamond. They weren't going to spend big money on repairs for vehicles that weren't going to be theirs in the long run. In fact, they had major problems getting ADL parts so you may well be seeing the product of a backlog of defects. Not defects that would lead to a PG9 but a culture of make do and mend meaning that they did enough to get them out legally and not much more.
Of the B7TLs that went on loan, it's not very representative that just because a vehicle was down there for a week and it didn't break down that they were somehow more reliable. As we saw when they had the three permanently, they too were often poorly; not surprising for 14 year old vehicles!
As with all things, Streetlites bother some people. The issues with brakes snatching is reported quite a bit though some of that is also down to the driver.
I've been on plenty of Streetlites and aside from a Rotala one in limp mode, I've not experienced a mechanical issue on any of them. I don't see them forever parked up on the roadside being attended to either, unlike the older B7RLEs that in summer 2018 were dropping like flies. The concern that FWoE had with them was as they exited warranty, a concern about how they might fare in middle age so hence sending them off to Scotland, Leicester, Wales etc.
There are undoubted issues and the gearbox mapping is one. Remember a 475** one of Bath depot on the 37 round Kelston and it was up and down (the gears) like a bride's nighty. However, Weston have seemed to sort theirs (and Hengrove too) and remember travelling on another 475** of Weston on the X5 and it was fine up and around Portishead and Gordano Valley. The ride quality is an issue as its not a heavyweight so you will feel more than an Eclipse. Generally, the construction isn't bad but I agree with
@D2007wsm about the emergency door handle. A design problem that just wasn't sorted. Otherwise, they're quite light and airy but the ride is a problem, especially the shorter versions.