Built-in Apache and Php is viable?
It is completely viable to use the built-in Apache and PHP on High Sierra.

From a high level point of view, the built-in Apache and PHP offers the same as Apache/PHP from HomeBrew. It is the same programs - and not a special "Apple made" version of Apache/PHP.

However if you want to customize the setup a lot by installing new modules, you will find that this is much easier with homebrew, as some of the files often necessary to build new modules have been removed with the High Sierra release.

Also if you want to be able to choose a specific Apache/PHP version rather than the default, then homebrew will allow you that.