So, I'm making a Windows 10 recovery drive (USB) as I type this and I'm wondering how good it is. I got the free Win 10 upgrade from 7 and I'm fine with it. But let's say I decide to do a clean install of Ubuntu or change my hard drive completely. Will the recovery drive still be able to reinstall Windows 10?

It should be good. I've done a clean install. You will need ProduKey to find your Windows 10 Key and save it. But this worked for me while Free upgrades is given. I don't know if they will change this key once the deadline passes or lock it out after? Also don't change any of your hardware.

So just to clarify, if I, for example, do a clean install of Ubuntu, wiping Win 10 out completely, I can still install Win 10 again after that as long as I have the recovery drive and my product key?

If you upgraded, the key is stored in the MS servers (note that a MS Account isn't needed for this), coded with you hardware. As long as you not make too much hardware changes, is supposed that you can reactivate your W10 copy when you like, because it submit you hardware hash to the MS servers in order to retrieve the activation key.

Edit: To clarify, I'm wrote "supposed" because for me it refused to reactivate in a new SSD, with the hazle that I'm need to make all the upgrade process again.

I believe you need to install windows 10 first and then Ubuntu after that. I read it somewhere can't recall it but windows first.