There aren't many limitations to what the Alexa Voice Service can provide, when compared to the official Echo units.
One major difference is that Alexa Voice service devices currently do not support Alexa-to-Alexa calling (sending and receiving) as of September 2017. This feature is only available to the official Echo devices (Echo, Echo Dot, Echo Show). This includes the 'Drop In' feature (which is often used as an intercom).
Messaging also seems to not be supported.
Some features are region-locked with the AVS. As of writing, the following are not available outside the US (i.e. in the UK and Germany):
- Traffic Reports
Skills and most other interactions are still available as normal with the Echo. In general, I'd expect that your home-build Echo, while losing some features, will still be pretty functional. If you have a display, you can also go beyond the original Echo's features by integrating Display Cards, which would normally only be available for the Echo Show.