You will need batteries with hydrogen fuel cells due to the fact the cells do not throttle quickly based on energy demand. The battery is required as a buffer for quickly discharging larger amounts of energy. So in effect you require a full blown electric drive train but with a smaller battery. In my opinion it is way less complex to just increase the size of the battery and get rid off the hydrogen parts.

The biggest problem with full electric powertrains is size of batteries and its weight. Keeping the size of batteries small and weighing less is a bonus in ICE car conversions, like I mentioned in my initial comment. Plus better replenishing time, which in most hydrogen fuel cells cars is around 3 minutes.

Edit: the Riversimple Rasa uses a hybrid lithium based battery/supercapacitor pack for this purpose.

With regards to weight - hydrogen tanks are heavy, even if the fuel is light. The tanks on a Toyota Mirai weight 87.5kg (193 lbs). In addition you need the fuel cell, piping, more wiring.. The actual weight advantage is not as great as it might seem.

I see people talking about refuellig time of hydrogen beein an advantage. Although most videoes I see on youtube seem to use ~10ish minutes - and it depends if the pumps has the correct pressure). Recharging an electric car is usually done in a few seconds of active time (plugging in the cable when you park), only if you exhaust the range during one trip you have to wait for recharging.

> Recharging an electric car is usually done in a few seconds of active time

You forgot the 5 minutes spent trying to download and install their own proprietary app, creating an account, just for the app to fail to start the charging...