After owning several ebikes, hubs are superior, I’ve never had one fail, but I have had two Bosch kid drives fail.... thankfully Bosch Warranty replaces them.
We have the Raleigh motus with the Bosch drive, it's great , powerful and quiet. Went out with some road bike lads and left them for dead on the hills! ;)
I take your point about battery cells but I personally would see that more a difference between doing your own e-bike conversion and buying a off the shelf e-bike from a UK or EU supplier. Even budget e-bikes are using good quality cells like LG, Samsung, Panasonic etc because complete e-bikes have a lot of EU certification to pass and that certification has to state which batteries are being used with possibly 1 or 2 alternative cell manufacturers as an option for a bike manufacturer. I think that is in the list of critical components and alternative suppliers. The Decathlon e-bike also on hotukdeals currently for £799 has high quality LG cells and a capacity of 420Wh slightly more than this bike. As for hub vs mid-mount motor reliability I don't think I'm stating anything out of the ordinary there, you only have to look at the internals to see how simple a direct drive brushless hub motor is, a geared hub motor is a little more complicated with its planetary gear system but still very simple but a mid-mount motor is super complex and often has to fit into a pretty small area. The previous generation of Bosch mid-mount motors had a huge amount of issues with plastic cogs and bearings issues. In contrast I've seen threads where people have used direct drive hub motors for many, many thousands of miles without issues. Direct drive hub motors have no moving parts except for the bearings of the axle and geared hub motors only have the 3 planetary gears assembly in the middle. This is the internal parts of a Bosch motor with a huge amount of components that wear out or fail including plastic cogs, a smaller motor, seals, bearings etc plus circuitry that has been known to fail either because of over-heating/over current or water ingress. I've heard of such motors failing within the first hour of use with heavier riders or people who have taken their bikes off-road. I don't think there is any surprise looking at those internals that long term the unit is destined to fail and with Bosch restricting supply of spares to official repair shops only plus charging huge amounts for those spares it can be extremely expensive to keep such motors working. I realise the Shimano steps system probably has more heavy duty and noisier metal cogs and spares are likely more reasonably priced anyway but still likely to have more problems than hub motors. You don't hear of so many problems with Shimano steps motors but that could be due to lower sales of such units. Not forgetting as well that many people with mid-mount motors get through a huge number of chains because of the extra power of the motor is delivered with the riders own power through the same chain so chains, cassettes, chainrings etc all wear out much faster even if you are lucky enough to get a mid-mount motor that does work long term without issues. The older Bosch motor had a small chainring that meant excessive chain wear and a short life but assume the chainring was user replaceable and obtainable although not sure about this. I realise the newer Bosch motors are improved in this regard.
So my experience has been the polar opposite of this: mid mount motor no issues (bosch), cheap and nasty hub motor = sad times. I can totally see the logic that replacing a hub motor is much easier than replacing a mid drive, which is integrated with the frame. 400wh is also a respectable battery size for last year's bikes from a reputable manufacturer (eg shimano, yamaha, bosch etc) at the sub £2k price point as they use good quality cells and proper battery management software. Mine has 400wh and I get ~100km per charge. A cheap battery is a false economy, if cell charge and discharge is not properly managed the life of the unit as a whole will be very poor. Different cells charge and discharge at slightly different rates and this needs to be damaged or the cells will die one by one if they get flat or overcharged because the management system is looking at the charge state of the unit as a whole not each individual cell. I also wouldn't have a 400wh+ no-brand battery in my house simply because of the fire risk. Something I'm going to use on a bike, with all the vibration and knocks that entails, plus the outside water, and then take it in my house and charge it? From a random factory in China, with no QA, no reputation on the line and no way to hold to account if it does pop? NOPE! I'm aware many people do use cheaper battery units but it's a personal cost/risk choice that for me and my family doesn't add up.
This is why I went with the Oxygen e bike in the end, I love the Bosch hubs but could see a problem rearing it’s head in a few years, plus they were a pain to de restrict, where the oxygen is a simple software lock