I wish we had a bunch of statistics for reliability. What's clear is that many of these simple bikes are ebikes (only) because of two core components, the motor and the battery. The Diamondback is a direct drive motor, but they are getting some serious output with a 48v battery. There isn't too much to a direct drive motor. It's not clear what the battery is, say LiFePo4 or LiPo, etc. The Volton is actually branding his battery, and Samsung does have a good rep. Volton has a bigger battery, which extends the life because the % drawdown is lower.
The Volton is a geared motor, which has more parts, so might be less reliable over a lot of hours. But geared is more efficient, normally, which again helps the battery.
Everything else is a bike. It's possible there are problems with the Volton frames, but we know all basic frames are made in either China, give or take. Court didn't find any real problems with the frame of the Volton, I assume. A lot of parts can be repaired by anyone who knows bikes.
If Diamondback is making a bike in China and importing it into the US, it's not all that different than when Joe imports a bike. In some ways, after a few years, he may have some edges. I'm sure if you asked Joe where he was getting his bikes, why he thinks they are good bikes, what he's learned, he might tell you. I am pretty sure if you asked Dback the same questions, a PR guy would spin a yarn.
I guess if i were getting close to $3k I would look for a cheap Haibike or something (online discounter) on the theory that a premium bike would hold value better, and Haibike puts you on the cutting edge. I personally stay away from the cutting edge, it might slice right through me.