Probably shouldn't go down this road with discussions of specific providers, but since you took the example of Drupal Gardens
I will give it to Drupal
Don't confuse Drupal Gardens with Drupal. These are very different things.
you apparently can't 301 inbound links.
On the test account I created, it had the Redirect module installed by default, which means that
- when I change a URL, the old URL is automatically set to 301 to the new URL
- it automatically captures 404s and presents you with a list so that you can set up 301s for them
- it lists current redirects and what HTTP status they send (301 or 302)
The only thing that's missing is that if you're migrating a site and for some reason you can't keep the old URLs (like I don't think URLs with GET params would work in Drupal Gardens, but I haven't tested it), you can't write general 301 using regular expressions and you can't just set a whole bunch of 301s in your httpd.conf or .htaccess. You would have to wait until a specific 404 gets triggered and then fix that one URL, one at a time.
Basic head tag functionality
On Pro accounts you have the Meta Tags module enabled which allows you to set
Open Graph Protocol
I'm not sure what more you can expect from a fully hosted, fully managed system that has to be able to roll out updates across thousands of sites automatically.
Clearly you aren't going to have everything with one of these hosted place.
With Wordpress and Drupal, you can't be letting regular users mess with the theme layer because in current implementations, they execute PHP pretty much without limitations. So in a situation like this you'll have limitations. Also, you need to keep the complexity limited to make it scalable. That said, in Drupal 8, the theme layer will have no PHP and then you get to the point where hosted sites like this can provide full design control to users. Whether or not they do so or how much they charge for the privilege remains to be seen, but it would be administratively a lot easier.
If you want to run a full Wordpress or Drupal install, then your SEO options are essentially limitless, but now you've moved into something that requires expertise and some dev resources. But hey, if you actually are getting to that level of optimization, you really shouldn't expect a fully hosted solution to be able to cater to very specific needs.
As with anything, you can't have the best of both worlds. Hybrid road/mountain bikes suck. No ski is great at touring and great at going downhill. And hosted solutions have their place, but yes, some sites will require a team of full time developers, sys admins, SEOs and so forth.