I purchase my Dyson V7 as an official refurb and the battery went in to lockout a few months after the short refurb warranty expired. Thus prompting the project. The vacuum is pretty handy for my small apartment, and when I bought it, I wanted something that had decent reviews and was from a real company, not just some Amazon seller with a bunch of random characters for a name. I still like the vacuum, I just wish Dyson hadn't designed it to fail.
I think there are two issues here: 1) The left out the cell balancing circuitry which is pretty standard for battery packs. 2) They designed their firmware to go in to permanent lockout when the cells go to far out of balance (300mV).
For the sake of discussion, I've heard interesting two arguments on why they might have disabled cell balancing: 1) Cell balancing could be a weak point on the ISL94208 IC they used, so they could have disabled cell balancing to actually increase overall reliability. We'd never know for sure, but I could see this being plausible. This is from someone on the EEVblog thread (https://www.eevblog.com/forum/projects/fu-dyson-bms-an-(unof...)
2) Cell balancing isn't actually useful because a poor battery cell will plummet in voltage under load anyways due to the higher ESR in the poor cell. At least, that was my interpretation of their argument after translating it from Russian to English in a comment on the YouTube video. While I see their point, especially in the vacuum's turbo mode which draws 17A vs the usual 3A, I think there would still be some usable capacity to be gained by balancing the cells even if one did have a much higher ESR.