Tesla is a unit of magnetic field strength. Hertz is a unit of frequency. Siemens is a measure of electric conductance. Try to find an SI unit that isn't also a company or product.

Trademarks aren't about owning a word, only about having exclusive use of it within a certain domain. If Instagram is successful, you will still be allowed to use the SI unit to quantify mass. You won't be able to market a competing product or service with the same name.

Common words have been under trademark-attack for some years now.

Carlsberg managed to register 'probably' in the context of beer and food:

https://tmdb.eu/trademark/008349227/eu/en/probably

and Specsavers recently won 'Should've' in the UK, to be referred to the EU level now, though thankfully they had their application narrowed-down to eyewear and medical devices.