Hello: Watts Industries would be a good place to start http://www.watts.com/ . They developed the temperature and pressure relief valve long ago.
Product manuals tell people what not to do. One may be able to assume this is in response to somebody getting hurt by doing something un-smart… ALWAYS unplug the toaster before washing it in the sink 😯 If you can reach folks in the engineering department of whatever company, they could likely get you up to speed fast on the safety of their product.
As to specific things to look for with instant heaters, I’d research how closely they control output temperature. There have been problems in the past with high temp spikes. How they are used can affect their performance.