Having tried and used almost all email clients that have been around, the best thing about Superhuman, until now, was that it made money by charging me a monthly fee, so I knew they can stay alive and continue without being sold to a bigger company and killed like many others before them.
Taking money from a VC is a bad sign for me. Call me old fashioned, but I would like to pay for what I use so they can build a business that lasts. VCs change the trajectory of businesses and usually what it means is getting bought by someone like Google and then being killed 18 months after that, leaving me holding the can.