Review (5/5): Great oral irrigator - read this for a list of the adaptor sizes
I purchased this SMAGREHO oral irrigator in March 2018. The water diverter is metal, as is one of the adaptors. The rest of the adaptors are white plastic. All of the adaptors have male threads on both sides. The threads on the diverter are female, and the threads in your faucet which your old aerator screwed in to need to be female as well. The metal adaptor fit our old (1970's / 1980's) Moen faucets.
I measured the outside diameter of the threaded part of each of the adaptors - the part that screws into your faucet. Here are the diameters: 0.68-0.69", 0.76-0.77", 0.83-0.84", 0.92" (this is the metal one), and 1.07-1.08". The other side of each of the adaptors (the side that screws into the diverter) measures 0.83-0.84".
I am impressed by the high-quality silicone gaskets that came with the unit. I experienced no leaks.
My faucets are the type where you pull the handle upward to turn it on. This makes it tricky to precisely adjust the water pressure, so the pressure is high enough to wash out food particles, but not so high it is painful to use. I suspect that facets which have handles that you rotate would give easier / more precise control of the water pressure.