For simple grid storage the efficiency is just bad. The electricity-hydrogen-electricity round-trip efficiency is about 60%, pumped hydro or batteries can do better.

So it's only worth it if you have other uses for the hydrogen or need longer-term storage, e.g. gas reservoirs.

Plus any infrastructure you build will have to be overprovisioned because by definition it'll be designed to absorb peaks and run idle the rest of the time, which makes it even less profitable.

Mostly because of the cost associated with turning electricity into hydrogen. Which I hope will be coming down in future.

Because of security. Hydrogen is pretty dangerous, much more dangerous than anything else.

Is security the reason that hydrogen vehicles haven’t taken off yet, or is it a question of infrastructure?

Well, security is fixable. It's hydrogen after all and not a heavy gas; in case of leaks it goes up not down. leaks are still dangerous as it explodes when being mixed with oxygen. the flames are invisible, you only feel being burned but you don't see it.

but the biggest problem is psychological: The Hindenburg, still

Just compare hydrogen cars with electric cars and you'll see why people prefer electric. CA has 33 hydrogen refueling stations, so many people in CA could get by with one.

Is it that much more dangerous than say LPG though?