Free subdomain or buy a domain?
What you give up, what you save and the cases where a name under does.software is the wrong answer.
This is a page from a service that hands out subdomains, telling you when not to use one. The honest version is that they solve a narrower problem than a domain, and that for that problem they're better.
What each one actually is
When you buy a domain you rent a name from a registrar and take charge of everything hanging off it, DNS records, a certificate, a mail server or a provider that runs one, and a renewal you can't forget. The name is yours in the sense that matters, which is that you can take it anywhere.
A name from here is a label under a domain we run. You configure what it does and we run everything underneath. You have no DNS records, you can't point it at another host and you can't take it with you.
Where a subdomain wins
Time. No registrar account, no nameserver change, no certificate, no MX record and no propagation to wait for. A name works minutes after it's approved, and HTTPS is already in place.
Cost at the low end. The free plan costs nothing and asks for no card, in exchange for renewing the name every 45 days, and the Starter plan costs from 2.56 euros a month. A domain isn't expensive either, but it's never free, and the first-year price is rarely the one you pay in the second.
The pieces you'd otherwise have to assemble. Mail forwarding, a static site, redirect rules and visit analytics come with the name. Doing the same on your own domain means a registrar, a host and a mail provider, each with its own bill and its own panel.
Where it doesn't
If the name is the brand. A company address that reads as a subdomain of somebody else's service tells every customer whose service it is. It's fine for a side project and wrong for a business you're asking people to trust with money.
If you're going to have to move. You can't take this name to another host, ever. The day you outgrow the service you'll change address, and you'll have to chase down every link, printed card and email signature pointing at the old one.
If you need DNS. There are no TXT records, so there's no domain verification for the services that ask for it. There are no records of your own of any kind, what the name does is what this service offers, and nothing else.
What it costs over five years
A .com usually runs between ten and fifteen euros a year, and that's the name alone. Add somewhere to host the site and somewhere to handle the mail, and a modest setup comfortably reaches fifty to a hundred a year once everything is paid.
On does.software the free plan costs nothing, renewing the name every 45 days, and we may give you a free 14-day trial with web hosting. The Starter plan is 2.56 euros a month if you pay for the whole year, 30.72, or 2.99 month to month, it takes the expiry off your name and brings a real mailbox. The Pro plan is 5.12 a month yearly, 61.44, or 5.99 month to month, and it raises every limit, three subdomains, twenty forwards, thirty path rules, three real mailboxes and 300 MB of hosting.
Moving later
Moving isn't hard, it's just public. Your content is yours and comes with you, the address doesn't. Count on keeping both for a while, redirecting the old name to the new one and letting the mail forward while you update the places that matter.
You have to decide which category you're in. If the name is going to end up on a card you hand to a client, buy the domain now. If it's a portfolio, a demo, a link for a talk or a contact address, the subdomain is the perfect tool.
Use a subdomain when the name has a job to do and doesn't have to be yours forever. Buy a domain as soon as somebody else's brand in your address could cost you a customer.