Arteta finished in 8th, after being appointed while they were in 10th.
Also, it's not just the position in the league that people criticize Ten Hag.
Arteta
8th, 56 goals, +8 GD, 47 xG (13th), xPts 50.2 (9th)
8th, 55 goals, +16 GD, 51.8 xG (10th), xGD +8 (7th), xPts 58.7 (8th)
5th, 61 goals, +13 GD, 60.5 xG (5th), xPts 64.7( 5th)
2nd, 88 goals, +45 GD, 71.9 xG (5th), xPts 72.5 (2nd)
2nd, 89 goals, +69 GD, 73.2 xG (4th), xPts 81.9 (2nd)
-Improvement every step of the way
-Performances equaling their results
Ten Hag
3rd, 58 goals, +15 GD, 67.7 xG (6th), 66.4 xPts (6th)
8th, 57 goals, -1 GD, 56.3 xG (10th), -12.8 xGD, +44.4 xPts (15th)
United result wise are actually doing better than what their performances actually show, which is what almost everyone that watches United will tell you via the eye test. That's not a good sign at all.
So United are scoring the same amount of goals as Arsenal in Arteta's 8th place side, should probably score 4 more than Arteta, but defensively United are conceding far more than Arteta's side, and are expected to finish far worse than they really did. So United tactically through their performances are showing that even though they are scoring as much, and should be scoring barely a hair over what Arteta's 8th place side did, are only able to achieve that mark by setting themselves up in a way that they sacrifice defensive stability in a terrible way.
I don't see how you find that acceptable. Even if you ignore the fact that United outspent Arteta by 200m euros (if you generously include the first season that Arteta was not included in the transfer period decisions), why would you believe in Ten Hag's set up that only gives you Arteta 8th place side attack, while giving United its Premier League 6th worst defensive side? Usually you can justify suicide football by the massive uptick in attacking performance. You don't get that with United's set up.
For example, Ange actually has Spurs with an expected goals conceded just below United's (14th best or 7th worst defensive side), but with the bonus of giving Spurs its 6th best attacking side in the PL (expecting 13 more goals than United). You can point to a clear benefit of his style of football. United really is just suicide football. Ten Hag is giving us midtable attacking output with relegation level defense. So on an entertainment level, United are boring to watch in attack, while making us feel like we just stole points away from the opposition.