Actually I think, that bringing WebOS to an existing device is more promising in the short to medium run, that waiting for a hardware producer to dare build a smartphone/tablet with openWebOS on it.
But if that Phoenix thing is successful will be determined by many factors, a few of them being:
1) (ofc) will they be able to bring openWebOS to existing devices
2) and if so: which devices? and how?
3) who will in the future provide services (i.e. profile, catalog, etc) for that solution? Atm it is HP, but will they invest money on the long run, if Phoenix stays the only way of bringing openWebOS to the market?
4) Apps, apps, apps. Will there be any? Will the old WebOS apps run? Will there be enough? Will they be good? Will they perform? ... etc
5) what is the business model for the Phoenix company? What do they have to do, to reach ROI? When will they win, when will they have to close down because of losses?
and so on.
As far as I see it, we can only speculate now. The idea seems sound, but there are too many other factors to deal with, to say as of yet, if they will prevail.