Cross-domain for marketing "services" Mautic servers/services

When configuring network infra for Mautic, WordPrress, etc. for a business venture is there any problems technical or otherwise with using a differnt domain (e.g. the form POST action) is in a different domain than the main website?

Is it generally better to use a subdomain of said domain?

ditto port numbers - is there any problems of running M3 sites on ports other than 80 (I thinking primarily about HTML form processing here)…