No Tankless Water heaters are not dangerous.
Actually Tankless Water Heaters are actually safer than water heaters that have A tank that builds up water pressure as it heats up.
A water heater that has a tank can actually blowup in some rare cases if the pressure relief valve is not working properly.
With A Tankless Water Heater you don't have to worry about the tank rupturing and exploding and Tankless Water Heaters are much better at providing instant and continuous hot water unlike the tank water heaters where you'd run out of hot water when it was all used up.