How about when you have any relevant information? For example after they show it in motion. Or, yes, during the beta, that sounds like a perfect time to decide that, e.g., scanning is too slow, too fiddly, doesn't hint at "interesting" places strong enough, general navigation becomes a hassle, … which may all be valid concerns, but there's zero information right now to form an opinion on any of those.
If you consider them valid concerns I'm not sure why it upsets you that they are voiced?
Very often humans need to be able to form opinions and voice concerns without all the data in the world on a given matter being available for them. Once ship has sailed, voicing em concerns becomes that much more futile.
Voicing concerns when something is in development= small change for it to have an affect.
Voicing concerns once everything is all done and being shipped for beta testing already..bit too late. By the time some open beta lands, the cake is already very much baked. Only concerns devs care of at that point have to do with bug fixes and polish. Whether it is a good idea to bake a cake by mixing salt and strawberries is a discussion already over and done by that point.