As a person who did a very large percentage of the work creating boot.initrd.systemd, I sure do have opinions 
We recently merged the PR to make systemd stage 1 the default. It’s an alternate implementation of the entire NixOS initrd, and the consensus has been for a long time now that it’s a lot better than the scripted initrd. It’s a lot more flexible, it has a lot more features, and it benefits a lot more from functionality provided by upstream systemd.