From what I understand, hydrogen fuel cells are very inefficient.
I guess it depends where you want to see the efficiency.
If you need to travel long distance, I have read that Mercedes say it takes a few seconds to fill up their hydrogen fuel cells whereas with Tesla even just partial charging takes 30 min on a special brand specific charging station, that gets you maybe 300km (since 30min won't give you full charge), in winter 200km? Then you wait again for half hour if you have a supercharger around? If you don't have supercharger but a regular charging station then it takes much longer. I have not driven a Tesla but this sounds to be a quite inefficient way to travel (say) 1200km to Lapland. When electric cars stop being a novelty and become common, you need a lot of charging stations to be able to handle the same amount of traffic as today with diesel and petrol cars, without slowing down the traffic.
If you're referring to the process of using electrical power (or worse, fossil fuel) to make hydrogen, then I suppose it is inefficient. With a hydrogen fuel cell car you have the option to charge the battery on the electrical network just like a normal electric car, overnight, or you can fill up the fuel cells and be on your way without waiting a long time. If the battery is of the same size then this should not be a problem (I guess a fuel cell car would have smaller battery, but this is a matter of priorities).
Today the most urgent need to reduce emissions is in cities where the air quality is poor and there are a lot of people breathing that foul air. Yes, it has been worse, but the air hasn't been cleaned as much as would be preferable, especially in big cities like London. However, if you are living in a a moderately sized city, you probably won't have access to a place to park your car so that you can charge overnight, unless you own a house in which case you are quite wealthy. Anyway most people in cities live in flats and that means there are no easy ways to charge an electric car. I don't believe there is much going on to solve this situation, which then opens up the opportunity to cars that use fuel cells because they can be filled up quickly, and natural gas because at least it burns cleaner than petrol or diesel.
From a CO2 perspective I suppose electric cars are better if the electricity itself is produced cleanly, which in many of the less affluent places of the Earth, it is not. But because of the lack of incentive to equip everyone with an opportunity to charge overnight, there won't be any fast switching to electric cars for the masses, I'm afraid. And to have a meaningful effect on climate and city air quality, it would have to involve masses of people switching.
