OK, I switched to using Google Contacts mainly because with Superhuman, you are forced to use it with Google Mail. When I was on Apple Mail, I actually used iCloud contacts (more private), but now I've had to import all my contacts into Google.
That alone was a chore because there are some limits on how many contacts you can move, but by chopping it into blocks, I finally imported everything (you export from Apple Contacts on the Mac into a series of VCF files and then import if you are doing this yourself.
It also turns out there is a synchronizer that makes your iCloud and Google contacts the same as well called cleverly Contacts Sync for Google Gmail and it appears to allow 2-way sync, so that's not a bad solution. They charge $8 and have a trial application too. This is actually not a bad solution.
The other one is that you can just completely flip to using Google Contacts for everything because Apple does allow you to have Google contacts acts as your default. What you are giving up then though are some of the privacy features. It's not a bad choice though
The move to Superhuman
Well, I've been using Superhuman regularly for the last year and while it takes some time to get used to, it is pretty elegant. There are two things that are a pain compared to say Apple Mail. The first is that it doesn't handle multiple accounts well. I have no less than 10 accounts that I monitor and Apple Mail let's me see them all as a single integrated email list. With Superhuman you have to ⌘-1 through ⌘-9 all the time and I often forget. But it is really nice to get reminders.
But, if you are using Superhuman that you have to use Google Mail and also Google Contacts, so this forced me to switch everything to Google contacts. Yuck! First, I like the Apple privacy features way more and second is the problem with Other contacts (see below..)
If you move to Google Contacts you have to deal with Other Contacts
One issue with Google Contacts though is that it has this notion of My Contacts, that is email addresses that you've added manually and then there are "Other Contacts" that applications and others thing add when you just send an email. Apple works differently, basically everything you've ever sent mail to goes into your contacts. That's pretty handy I find.
So with Google, I found that over the last two years, over 10% of the addresses I have are not in my main contacts. That's kind of a big deal because it means that applications like say, Apple Calendar, can't see other contacts, so you have to manually add them. Basically, if you go to contacts.google.com you will see a list of contacts, and way at the bottom is "other contacts". If you select one, you get an icon that is a person with a plus sign, click on that and it adds them to the main list.
You probably want to curate the list though since there will be bogus things. The other annoying thing is that if you select All and you have a lot of contacts, the Add People icon disappears. So now you do have to spend a bunch of time curating your Other Contacts. Sigh.
And at least when I click on 400 contacts, it will say working and it takes minutes for it to happen.