Do you think a FQD domain name matters?

I'm wondering if not having a custom domain matters. Having a custom domain seems more "professional" and helps(?) for SEO, but I don't care if a good product/solution is part of something like Netlify or Vercel.

Since platforms offer suitable solutions for rapid development and deployment, I've been tempted to use a subdomain name instead of buying another domain, and purchase one if MRR allows it..

What do you think about it?