I feel like you're putting an expectation for United that is very much on the high end of the spectrum against one for Dortmund which is on the lower end for them. A similarly high-running expectation for Dortmund would sound more amongst the line of "be indisputable #2 in the league with a good chance of challenging for the title, reliably reach the Ro16 in the CL and have at least a decent outsider's chance of making final, maybe even winning the thing".
A more... realistic expectation for both clubs in the medium term is pretty similar, I'd say: reliably establish oneself in the direct CL qualification spots, have a shot for the title if maybe the stars align just right, regularly make it into at least the knock-out phase if not direct qualification to Ro16 in the CL.
If that is how you feel, that is probably because I wasn't precise in what sense I used the word 'expectations'. The better word would probably have been 'ambition'.
And then, the disparity between ambition and realistic short term expectation is much more stark at Man United than it is at BVB, where both basically align - the difference between
be indisputable #2 in the league with a good chance of challenging for the title, reliably reach the Ro16 in the CL and have at least a decent outsider's chance of making final, maybe even winning the thing
and
reliably establish oneself in the direct CL qualification spots, have a shot for the title if maybe the stars align just right, regularly make it into at least the knock-out phase if not direct qualification to Ro16 in the CL
really isn't that big..
Where as at Utd, the ambition of being the top club in the league and a top ~5 club in the world is starkly disaligned with the somewhat realistic short term expectation of contending for top 4-5 with the likes of Chelsea, Newcastle, Villa.
BVBs ambition doesn't massively exceed their realistic expectations, and that is substantiated by how they're financially in an easy position to match those ambitions. They are undisputed 2nd in both turnover and wage bill, with a big lead on Leipzig and Leverkusen, not to mention Stuttgart or Frankfurt.
33% more turnover than Leipzig, 50% more turnover than Leverkusen. 20% higher wage bill than Leipzig, 40% higher than Leverkusen.
(
https://media.dfl.de/sites/2/2024/05/Clubs-der-Bundesliga-2024-25-Geschaeftsjahresende-2023.pdf)
Utd are also 2nd in turnover, but much closer followed by Liverpool, Chelsea, Spurs. The wage bill is the 4th highest, Chelsea, Liverpool, City paying more. And all that to a level where basically even Aston Villa pay more in wages than any Bundesliga club bar Bayern.
Financially the PL is much more competitive at the top.
https://www.bbc.com/sport/football/68713522
Then, let's not pretend as if BVB's squad building and transfer business over the years, if underwhelming, has been anywhere as catastrophic as Man United's.
As a BVB manager, if you're a competent manager you are in a good position to match the ambition of the club. At Utd, as a competent manager, you are in a precarious situation to match even the expectations, while the specter of the club's ambition still adds to the pressure.
And that is precisely what makes the Man United job so difficult and the BVB job so easy. The air is thinner here, to fail to recognize that is to fundamentally misunderstand the predicament the club is in. That Ten Hag would find much more favourable and benevolent conditions at BVB is without doubt to me.