I'm reading some reviews that claim the difficulty is too hard for the expansion. Is this true? Asking people who own the expansion and have played the most recently version of it regularly. Thank you!

I waould say definitely not.
One factor is that there are more low difficulty flashpoints in this expansion.

Battletech isnt really hard at all, would call it quite easy.. With Urban Warfare, some tactics have to be adopted for ECM enemies (Ravens). 1.7.0 had a bug with randomization. This caused the game to generate always the same mechs. In endgame its quite nice, as you got missions with 3-4 Atlases which gave lots of cash. In starting game it gave misions with 4 Firestartes, which were extremely hard.