To not get this discussion all fixed on that playoff series, here some stats about Simmons (ánd Tatum) over their games this season against good teams (unbiased + .500 teams so far).
Ben Simmons (Philadelphia 76ers, 6-6 against + .500 teams)
Per 36 min: 15,2 points / 10,2 rebounds / 7,7 assists / 1,6 steals / 0,8 blocks / 3,9 turnovers
Shooting percentages (57 / 0 / 49 - 57 TS%), AST/TO (2,0), +/- (- 2,3).
He played exactly 36 minutes on average, so this truly are his stats over those 10 games. They aren't much different from his regular play (though 2 scored points less). Rounded he averages: 15/10/8 on 57 TS%, 2,0 AST/TO, which looks very impressive. Simmons didn't take any 3-point-shot by the way.
However, Philadelphia didn't play that well, in fact their record against the top in the East (Toronto, Milwaukee, Indiana and Boston) is currently 1-5. Philadelphia gets outscored by 2,1 points against + .500 teams. Surprisingly with Simmons on the court they do even 0,9 points worse. In total Simmons has a plus/minus of -27.
Jayson Tatum (Boston Celtics, 6-6 against + .500 teams)
Per 36 min: 19,2 points / 6,8 rebounds / 2,6 assists / 0,8 steals / 0,9 blocks /2,1 turnovers
Shooting percentages (45 / 40 / 89 - 57 TS%), AST/TO (1,3), +/- (+ 3,4).
Standardized stats, but Tatum also plays a hefty 33,1 minutes, so not much bias. His scoring is exactly the same as it is against other teams and overall no real difference to notice. His rounded stats: 19/7/3 on 57 TS%, 1,3 AST/TO. His rebounding is less good than Simmons and obviously not a playmaker, but he had 5 games with 20+ points against Simmons none.
Boston's record against + .500 teams is the same (6-6), but they won their games on average by 1,1 point. With Tatum on the court they do even 2 points better. In comparison with Simmons the net rating from Tatum is 5,7 points better. This is quite spectacular.
The general view is that Philadelphia lacks depth. And this certainly shows in advanced stats, where the bench is a net negative on the court and all the starters are huge positives. Yet Simmons' rating is clearly the worst of them. Which is weird, since Simmons plays the majority of his minutes together with the other stars, but for instance against + .500 teams his net rating is 5,2 points less good than Embiid.
Does this mean that Simmons can get good stats as a primary playmaker, but that his play doesn't really yield winning games? Is his lack and willingness of shooting outside the paint so impactful that it gains an overall advantage for the opponent? Is the problem simply that Embiid and Simmons can't play together?