We have one important tip for this!
Make sure your website is up-to-date. We often come across websites that are still using very outdated versions of WordPress or Joomla, or plugins that are heavily outdated. The result is that your website is vulnerable to hacks and external attacks, but at the same time there is a risk that your website is not suitable (anymore) for new versions of, for example, PHP.
When your website and all plugins are properly updated, the chance of a migration failing is very low.