I would definitely read a lot of reviews of the host before going with them. There have been hosts I was with for 5+ months where everything went great for me and then I started having the same issues others had already written about prior.
One thing I can tell you for sure is most free sites are more than good enough for flat-file site hosting. The issues almost always come in when you are doing heavy PHP/SQL on your site (for example, forums or CMS's).
Both of those also have flat-file versions, so that's really not an issue.
So really it all boils down to what you want to use the host for:
1) Are you looking for something reliable? If so, don't use them.
2) Are you going to use a lot of PHP/SQL? If so, and the site is going to be *live*, it is not suggested to use them.
3) The domain/subdomain thing is not an issue at all. You can use CPanel to link up a domain you purchased elsewhere.
4) If you are just testing various things or are starting out fresh, I would suggest to use them for that purpose. Even with PHP/SQL, there aren't a lot of issues as long as there aren't many visitors.