The problem is the query - how do you find users with stringProperty = stringTwo? You also can't define an index on this kind of column.
The expandos apply to pretty much all Liferay entities. As frameworks go, it provides a consistent interface to expand any liferay entity without actually touching the liferay entities themselves.
The expando tables may be over-normalized, but it does support creating queries to join user against the expandos to match on specific values. Indexes on the expando values simplify search, both from a database perspective as well as from a lucene/solr/elastic perspective.
And I would argue w/ the good performance pro. It may be easier for adds/updates, but 9 times out of 10 you're going to be reading data from the database. You're going to be querying, you're going to be retrieving, and you're going to be indexing. A single json string column is going to be a penalty for you here.
Say for my json column I have favoriteFood and foodAllergy attributes but I want to find every user with a nut allergy. You cannot form a generic SQL query that would select all users that have "nut" in the foodAllergy item but not those that have "nut" as favoriteFood.
But I can handle this easily using expandos.
And remember one of the project rules - we don't care about developer pain. Seriously, if it takes a developer 4 hours for a json column or 8 hours for expandos, who cares? The only thing that matters is that we deliver the correct results. So the expandos might be the more involved development effort, but it will lead to correct results.
