If tactile controls don’t bother you (I mean if we compare with physical faders and knobs), the iPad can be a decent alternative. Avid has its own app for Protools and for Reaper I recommend TouchOSC : it’s easy to setup and you can customize the layout as you want.
There are also apps like Quadro if you want to add shortcuts and/or macros for specific workflow.
The issue with this kind of solution (imho) is the lack of physical feedbacks. You have to constantly look at where you put your fingers on the tablet so it’s not really comfortable. However it’s better than nothing if you are on the go.
Another solution if you prefer physical controls might be the Behringer’s X-touch controllers but it’s of course less portable.
The Pro Tools app for iOS is eucon compatible. If you install the eucon driver on your computer, you can use the PT app to control anything that supports eucon surfaces. Reaper does. I use the PT app on my iPad as a control surface for Reaper when I’m traveling.
Metagrid is a handy tool and worth a look. I’ve got all my favorite Reaper actions on it so they’re easily accessible while I’m working.
I was going to buy one of Avid’s small mixers but I am so glad I changed my mind to an iPad Pro.
I have used both the Logic Remote and PT | Control. Both work great and I haven’t had too many hiccups with them yet.
There is an excellent app called DUET that let’s you make your iPad into a second monitor. The connection has absolutely no lag, haven’t had any annoying problems with it, and you don’t have to update it constantly. This is great when you are editing sound to picture, Protools on your computer with the video playing along side on your iPad – Or – you can have your edit window on your computer and mixer on your iPad / ViceVersa.
To address your two concerns:
Productivity is great, I’d say far greater than any small mixer and / or controller surface since it allows for multitasking and that your DAW doesn’t crash.
Portability – Even if I have the iPad Pro version, it still fits into smaller brief cases/ small backpacks. It’s nice and thin. I got and iPad case along with it too, I can lay it horizontally or upright. (another must-have)
It definitely a worth while investment for all your sound work!
Other uses for the iPad, you can use it as a note book, calculator, sound design tool (apps) and with the other billion apps. I am on vacation now and I only brought my laptop, iPad Pro and my headphones to edit and do some mixing.