I'm testing out Allo now. For reference, my main number is with Google Voice and I use Hangouts for texting. This set up allows me to have multiple cell phones ring when people call me, and I can text or call them back from any phone as long as I logged into that device with my Google Voice/Hangouts info. I can also text from a laptop/desktop using Hangouts. To the person I'm calling/texting all they see is my main number.
Allo needs to be able to take over for Hangouts and do all of the above, but right now it doesn't.
First issue - the sms fallback using the sms relay number is pretty dumb. Google probably did this to avoid having texts disappear into the void when uninstalling Allo or switching devices like how iMessage does.
Second issue - no multi device support is also dumb. I installed Allo on one phone, sent a couple messages to some friends so they can install it. Grabbed my other phone and also installed Allo. Both phones are using my Google Voice number, so currently any text I receive or send I can do it from either phone using Hangouts. With Allo, activating the 2nd device will cause it to disconnect from the first device. The 2nd device loaded up the contacts that I had sent messages using the first device, but the actual messages didn't load. I could live with no multi device support if messages were backed up in the cloud, i.e. forced to log out of one phone if another device logs in but all your messages carry over.