In reply to David Hayes.

I use Bedrock & Composer, and love it, but I just want to reiterate this point from Scott:

"Anyone can use Composer to install WP core/plugins AND still be able to update/install either from the admin. Which is why if a developer wants to start a site with Composer and then hand it off to the client, it will work exactly as a normal WP install would (provided they include all the package sources along with it)."

There really isn't any downside to using Composer and handing it off to an end user. You can manage the site and deploy using it and the user can still expect things to work "The WordPress Way" if you stop. Heck, I update plugins using the Admin panel if there was an important bug fixed (recently updated BackWPUp because it's Dropbox integration broke from the panel, actually) and update Composer later.

So quite frankly, I don't see there being much of a drawback even for people quickly spinning up a site and handing it over to an end user. The only thing I'd say is it doesn't really make sense to use it if you're still new; you should be learning how to develop, not fiddling around with tools (and I'll admit, early in my learning, I spent too much time fiddling around with tools), but I think that can be said of a lot of things, and it's not really particular to WordPress development so much as it's a programming thing more generally.

Learn the basics before you dive into tools like this.