The SAS are heroes, but they aren't necessarily nice people. This dramatized version of the story of the role of the SAS in WW2 has flaws but those are worth ignoring. A reality hard to swallow for modern audiences is that flippancy and a lack of respect, the notion of war as a great adventure was part of the SAS ethos at the time and indeed in the British armed forces generally. But alongside the flippancy was serious professionalism. The SAS excelled at desert warfare especially the long range navigation across the desert needed to pinpoint German positions. They invented "shock and awe", but didn't have a name for it, with hit and run attacks bringing overwhelming firepower and a knowledge of how to use that firepower effectively that was a revolution in warfare at the time. This aspect of the SAS is portrayed well.
The attempt to inject love interest into David Stirling's role in the formation of the SAS is clumsy and seems unnecessary.
The language and attitudes portrayed are of their time, particularly a casual dislike of the French and Australian combatants. It would have been wrong to dress this up for modern audiences. However I suspect that these attitudes will irritate many viewers.
Watching the related documentary series "SAS Rogue Warriors" is recommended, it explains elements of the plot that are not obvious such as the key role played by people who are minor characters in the drama.