The belt size is more than enough -- I've never tripped, never felt unsure footed. I'd prefer it to be smaller actually, so it had a smaller footprint.
I used the smartphone app very rarely - set the default speed at 3 km/h. I exclusively use the manual mode -- this is the only one that makes sense for a walking desk. I usually walk at 3.5 or 4 km/h, sometimes faster if I want some exercise.
The biggest problem for me is the noise the fan makes. This is the fan that cools the engine. It only switches on a minute or two after you start walking and then stays on until the treadmill stops. At lower belt speeds, up to 4km/h, it is noisier than the belt and the engine themselves.
At 3.5 km/h the treadmill is 50-51 db before the fan switches on and 52-53 db with it. The fan noise is somehow more annoying too. It is tolerable for me, but it bothers my partner when I'm using it. It mostly limits the time I'm walking to when she is away. I sometimes get away with using it when she has noise canceling headphones on.
If there was a walking pad running 3.5 km/h at 47 db I'd replace this one in a heartbeat. I would also consider one that has at least a quieter fan.
A minor problem with the treadmill is static electricity it creas on the user. I'm not sure what is causing it. I've never experienced it from gym treadmills, and it was not happening at the beginning with this one either. Now I need to be grounded or else I experience uncomfortable static discharges when touching a laptop.
Overall the benefits are bigger than the drawbacks.
