Running a Debian installation on Android as a NAS / VPN - what would I lose as compared to a Raspberry Pi or similar comp?
Disadvantages

I'm assuming that you already know the advantages, so I'll just tell you the disadvantages:

 Hardware errors: Many users have reported this. Issues include (but aren't limited to) WiFi connection errors, Bluetooth not working, Hardware buttons not working, etc. (These obviously vary from phone to phone)

 OS errors: Some users have reported that Linux hangs randomly and that the phone needs to be forcefully shut down by long-pressing the power button or taking the battery out. (These depend on which distro you are using)

Software errors: A friend of mine says that some software doesn't work the same way it would on a PC. You have been warned :/

Driver errors: Some hardware may not be detected by linux. This depends on the distro you are using and the hardware support provided by it.

Hardware Comparison

Here's a comparison:

-------------------------------------------------------------------------
|Criterion              |Android (Android One)  |RaspberryPi            |
|-----------------------|-----------------------|-----------------------|
|Connectivity           |                       |                       |
| (a)Bluetooth          |Built-in               |External Dongle        |
| (b)WiFi               |Built-in               |External Dongle        |
| (c)SIM - 2G           |Built-in               |External Dongle        |
| (d)SIM - 3G           |Built-in               |External Dongle        |
|-----------------------|-----------------------|-----------------------|
|Specs                  |                       |                       |
| (a)RAM                |1-2 GB                 |1-2 GB                 |
| (b)CPU                |1.3 GHz                |1.2 GHz                |
| (c)Storage            |SD card + internal     |SD card                |
| (d)SIM Slots          |2                      |None                   |
|-----------------------|-----------------------|-----------------------|
|Battery                |Internal               |External               |
|                       |(Fixed Capacity)       |(Your Choice!)         |
|                       |(Use Power Bank)       |                       |
|-----------------------|-----------------------|-----------------------|
|Cost                   |INR 6999-8999          |INR 1299-3899          |
|                       |USD 100-150            |USD 20-60              |
|                       |                       |(+Extras)              |
|-----------------------|-----------------------|-----------------------|


Conclusion

So what does all that (^) even mean? According to me, it's worth a try. If it works, congratulations, you are a very lucky man (or woman). If it doesn't, take it as an interesting experience...

TL;DR: Try it, but don't expect too much...

PS: 
The are plenty of ways in which this can go wrong [see https://github.com/meefik/linuxdeploy/issues ]

A tutorial and some troubleshooting tips can be found at http://null-byte.wonderhowto.com/how-to/rooted-android-your-new-pentesting-tool-0159093/