Apart from an IR test, also check that there is a proper overheat cut out (manual reset thermostat). IIRC it should have by that time but always worth double checking.
As to "can anything catastrophic happen, in theory yes - but in practice the safeties would take care of it.
First line of safety - the overheat cutout in the immersion. Should that fail, then the water can boil - boiling water in a sealed container = big bang
But, there should be a pressure relief valve AND an overheat dump valve (commonly in one unit) - they are a requirement for unvented cylinders, but you never know if it was properly installed in the first place and if it's been properly maintained since. Either valve will avoid the big bang - one by relieving the pressure, the other by dumping water and thus allowing fresh (cold) water in.
Mythbusters did a show on this - electric water heaters are more common over in the USA I believe. They had to do some work to stop th safeties working, but after that they got one to blow itself out through the roof of their mocked up house.
As I said, if you can find the whole thing, they had to do some work to stop the safety devices working. But if they do stop working, you have a bomb in the making.
The failure mode is : temperature keeps rising, pressure keeps rising, temperature gets to well above 100˚C, the pressure eventually causes the cylinder to fail (in the video above, the join between sides and bottom fails). You now have a cylinder of superheated water which flash boils into steam which then propels the cylinder like a rocket and you don't want to be anywhere near the blast unless you enjoy a flash poaching. Lookup BLEVE - boiling liquid expanding vapour explosion.