Anyway, my more detailed overall thoughts:
On the plus side, it may encourage more people to post there sightings of birds not considered established, and will hopefully convince reviewers to not dismiss them out of hand. A major issue, at least earlier in Ebird's spread, was reviewers flagging all birds that were considered established. Also I don't have a real horse in this rate. While I religiously post my birding trip sightings to ebird, I've never used it for keeping track of life/state lists. So they can flag away all they want.
Anyway, my major concerns would be as follows:
1)The provisional classification over-complicates the situation rather than resolves it. My assumptions and hopes is that it would reduce the endless arguing over whether something is "established" or not, by producing a inclusive middle-ground that populations of introduced birds not yet deemed established could slot. However, it's just added another thing to argue over. Not only can we debate whether X is established or not, we can now debate whether it's established enough to be provisional! I would much prefer a simpler criteria. Any discrete population that is present in a given region, showing evidence of breeding and replacement, for X amount of years, or clear population expansion. That's its. Instead, we have another nebulously defined category, that really seems to be "These should totes be countable but the checklist committee hasn't gotten to them yet".
2) Consistency. As you say, there is consistency in how ebird folks/locals view non-native species. But I don't think it's just a matter of local knowledge. I think there are also biases and just differences in how people approach non-native species.
As an example, lets look at one species considered Provisional in Hawaii, Tanimbar Corella, and one species not considered Provisional in California, Black-throated Magpie-Jay.
The corella has a few records from the Oahu, but is largely reported from the Big Island. These are all clustered in a small area around Kailua-Kona. On ebird, there are no records older than 2018, and the high count for the species in a single checklist is 7 individuals.
Now lets look at the Magpie-Jay. The population of these birds is clustered in the Tijuana River Valley south of San Diego, on the U.S.-Mexico border. On a quick perusal on ebird, the High count on any given checklist is 12, and the oldest record is 2004.
So why the difference in status? How much of this is actually down to the different attitudes towards introduced species in Hawaii vs California, the latter a former stronghold of the idea that NO introduced species should be ever be "countable".
3) The lumping of potential vagrants in with potentially established non-natives. These are two different categories of occurrence. No one seriously thinks the Striped Sparrow or Red Warbler are going to found a new colony, just like people don't assume every other vagrant will immediately lead to a new population. These are origin hypothetical; It's uncertain whether they may represent wild vagrants or human-assisted species, either critters that escaped from captivity or were accidentally moved by getting stuck in a truck or something. I don't think its useful to lump these categories together. A provisional introduced species exists as a population that may or may not grow in time and expand there distribution. A uncertain vagrant may have new information that either indicates a past record is a wild bird, or a pattern of occurrences or new behavioral information that suggests vagrancy is more possible. Completely different criteria for acceptance should warrant completely different categorization.
