Cockpit is okay but it's basically Red Hat's equivalent to the Windows Server Manager tool, and I have no doubt it was directly inspired by Server Manager. It's development and improvement over the years has been painfully slow.
Nobody who is comfortable with an ssh session uses Cockpit, except maybe to create new VMs, and even then all of these comments comparing it to Proxmox are just whack because it doesn't have a quarter of the features the Proxmox UI offers. The utility for managing VMs is a recent development and even then I still prefer the Virtual Machine Manager tool because I don't want to deal with the latency increase and other limitations of working through a browser.
But anyway, there's a ton of things you can't do with Cockpit, and never will be able to do. It's for people who want to point and click, can't do a bash for/while loop, don't understand pipe chaining commands, and don't like using vim.
Like happyweasel said, it's basically webmin for Red Hat.
It's kinda cool, but it's so old now and development has been so slow and it's been so over-hyped that I don't pay attention to it at all and I've never used it except what was required to get certified.