a subdomain or your own 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 on 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 does, and that for that problem they are better.
what each one actually is
When you buy a domain you rent a name from a registrar and take on everything under it: DNS records, a certificate, a mail server or a provider for one, and a renewal you must not miss. You own the name in the sense that matters, which is that you can move it anywhere.
A name here is a label under a domain we run. You configure what it does and we run everything underneath. You get no DNS records, you cannot point it at another host, and you cannot 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 out. A name works minutes after it is approved, and HTTPS is already there.
Cost at the bottom end. The base service is free and needs no card. A domain is not expensive, but it is never free, and the first-year price is rarely the price you pay in year two.
The things you would otherwise assemble. Mail forwarding, a static site, redirect rules and visit analytics come with the name. Doing the same on a domain you own means a registrar, a host and a mail provider, each with its own bill and its own dashboard.
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. That is fine for a side project and wrong for a business you are asking people to trust with money.
If you need to move. You cannot take this name to another host, ever. The day you outgrow the service you change address, and every link, printed card and email signature pointing at the old one has to be chased down.
If you need DNS. No TXT records, so no domain verification for the services that ask for one. No records of your own at all: what the name does is what this service offers, and nothing else.
what it costs over five years
A .com tends to sit around ten to fifteen euros a year, and that is the name alone. Add somewhere to host the site and somewhere to handle the mail, and a modest setup is comfortably fifty to a hundred a year once everything is paid for.
Here the free plan is zero. Starter is 2.56 euros a month, charged once a year as 30.72, and takes the expiry off your name. Pro is 5.12 a month, 61.44 a year, and raises every limit: three subdomains, twenty forwards, thirty path rules, three real mailboxes and 300 MB of hosting.
moving later
Moving is not hard, it is just public. Your content is yours and comes with you; the address does not. Plan on running both for a while, redirecting the old name at the new one and letting the mail forward while you update the places that matter.
The cheap insurance is deciding early which category you are in. If the name will end up on a business card you hand to a customer, buy the domain now. If it is a portfolio, a demo, a link for a talk or a contact address, the subdomain is the right size of tool and you can stop thinking about it.
Use a subdomain when the name has a job to do and does not have to be yours forever. Buy a domain the moment somebody else's brand in your address would cost you a customer.