I'm sure he's doing it to be contrarian to his perceived enemies, "CDPR bad, Rian good!"
The only good thing I can say about Last Jedi based on the screenshots he tweeted there is that the set designers tried and gave the viewer a really interesting spread of scenes. Visually that salt planet getting red scrapes by the vehicles is really interesting and eye-catching, as are Snoke's throne room and a number of other locales. It's just the writing, acting and choreography are so bad they steal the show. Some of the people working on that movie were really trying, it's a shame.