Dynamic DNS
Dynamic DNS points a subdomain you claimed in a web form at an address that changes. An updater on the machine pushes the new address when it moves, which is the problem it was built for and still solves.
Claimed by hand, against an allowance
A subdomain there is claimed by a person in a browser, out of a fixed allowance. That is fine for a handful, and it is the wrong shape for anything that creates names while it runs: there is no call to make, and nothing hands a name back when whatever needed it is gone.
A minted hostname is created and released over the API, and how many you hold is what the plan says rather than what the form allows.
Certificates for more than one machine
Every hostname can return its own DNS-01 credential, scoped to that one hostname. A fleet renews together rather than in turn, and the machine that renews one certificate cannot touch the rest.
What the Public Suffix List does and does not buy
duckdns.org is one registrable domain and everyone is on it. It is on the Public Suffix List, so cookies and certificate rate limits stay yours. Reputation does not: a blocklist applies to the registrable domain, so it lands on every label under it whoever earned it.
With DNSMint the domain is yours alone, and on a plan holding several you decide which names sit on which.
The quickstart goes from an address to HTTPS, and the other comparisons cover the rest of what people use instead.