So, I did some crude reliability stats based on the info available on Bustimes.
I based it on the number of days that the vehicles were in service over the last 31 days, obviously it doesn't take into account the days were the PVR is reduced (ie. Sundays and bank holidays), but I also did a comparison excluding these days as in theory all vehicles should be in service. If a vehicle was only out for half a day, I counted it as half a day etc.

Of the 67 plate B5s, the average 'reliability' was 79%, or 90% if we only include days with a full PVR
For the B5 spares, the average 'reliability' was 68%, or 78%

It breaks down per vehicle thusly:
6308 - 77% - 92%
6309 - 97% - 100%
6310 - 61% - 72%
6311 - 80% - 94%
6312 - 84% - 96%
6313 - 75% - 88%
6314 - 79% - 90%
6334 - 73% - 82%
6335 - 63% - 74%

In the last 31 days, there have been 6 days where all B5s (including spares) have been in service, 10 days with one out of service, 7 days with 2 out of service, 1 day with 3 out of service, and 1 day with 6 out of service.

6310 has been out of service since the 14th April, but other than that, the longest a vehicle has been out of service was 3 days (with one of those days being a Sunday, so it could well have been out of service due to the reduced PVR)

As a comparison, I picked a new StreetDeck (6369) and E400 (6342) at 'random' (nothing to do with the numbers), with the StreetDeck being 74% reliable, and the E400 being 81% reliable. I couldn't be arsed to do a larger sample of them, so that's probably not that representative.


So, taking all that into consideration, I'd say the B5s are actually, on the whole, fairly reliable on the X21, with most of the issues stemming from the lack of branded spares other than the ones forming part of the PVR. 
If the X21 had 2 extra branded vehicles (like it's supposed to), then there would have only been 2 days in the last month that there would have been non X-lines vehicles

If I have some free time next week, I might calculate the reliability stats on some other vehicle types, maybe not doing a full month on them because I'm lazy, but I imagine checking 2 weeks worth would probably be enough