Sadly the main drawback seems to be cost; as is evident in the pricing of the startup's product. More than that, until gasoline vehicle externalities are properly priced in, the cost of any alternative will be exorbitant in comparison.
There have been some concept vehicles, and larger commercial vehicles; but they never caught on. It seems like electric will win before diesel/electric has a real chance. I always thought it'd make the most sense on motor homes and the like, where people tend to bring a small generator for electric appliances anyway.
So in general, the disadvantages of a diesel-electric system outweigh any advantages when you take a look at how trucks operate.
New trucks have "automatic" transmissions (not torque converters- they're more like automatically shifting standard transmissions), so no manual shifting required.
Somewhat related, but refrigerated trailers ("refers") often use a diesel-electric system because it's easy to plug in to shore power and cut the diesel engine.
Pure electric just won't work for any sizable portion of trucks getting converted, grid requirements just can't be met.
Diesel Electric transmissions in trains are in the 80's percent efficient range, double conversion from mechanical to electrical back to mechanical has higher losses. Trains make up for this with lower rolling and aero resistance, they need the precise traction control of the electric transmission to prevent wheel slip along with the impracticality of mechanically routing power to the wheels. Semis do not have this issue.
Even if trucks do spend the majority of their time on the highway, it's not all of the time and the efficiency gains every time they need to stop and start are significant.
The biggest problem isn't the truck itself though, it's charging it. A single truck requires as much energy as a dozen cars meaning that a truck fast charger is going to need the equivalent of an entire bay of car fast chargers, and they require charging significantly more often. A car can get 95% of it's charging done at home overnight, whereas a battery electric truck is likely to need fast charging several times a day. The most convenient place to put these chargers would be at the loading bays where trucks pick up and drop off trailers, but these are generally not owned by the trucking company so the owner has little incentive to drop hundreds of thousands or even millions of dollars on chargers to save the trucking company money _sometimes_.