Maintenance free sealed batteries have been around for years and years.

They are extensively used in UPS and inside building applications.

In Australia the vast majority of vehicle batteries have been sealed maintenance free for many years.

In 40 years dealing with DC power supplies and all sorts of batteries - sealed and unsealed, I have yet to see a battery explode except for exceptional circumstances.

The sealed battery production is a massive industry. Hybrid cars, Computer room back up power supplies, solar and wind power application. Small home UPS equipment. The list goes on and on. Most of the industrial applications are meant for batteries to be installed inside a building. There are strict building codes that apply to the installation of the older open venting lead acid batteries.

One observation from sheer experience which will be disputed by the non engineering based marketing people. Maintenance free batteries do not last as long as the older lead acid open types.

My '93 W140 had a sealed battery in the boot. W210 and W209 both had sealed batteries - all OEM