Engineering is harder than Letters and Sciences at every single UC, but the disparity is the greatest for the top two - UCB and UCLA.
As an example, looking at the transfer admission profile, we see an average accepted GPA of 3.87 for engineering but 3.69 for L&S, even though the engineering applicants had to take harder classes like 4-5 math classes, 2-3 physics classes, 1-2 chemistry classes, etc. While that may not seem like a huge difference, keep in mind that a student with a 3.87 GPA has lost only 0.13 GPA pts, while the 3.69 student has lost 0.31 which is more than twice the amount.
https://www.admission.ucla.edu/prospect/adm_tr/Tr_Prof14.htm
As another example, Berkeley’s average UC GPA for engineering majors was 4.46 in 2011, back when the overall average was around 4.25. That 0.2 GPA difference is about the same as the difference between UCB L&S and UCSD L&S. As you said, engineering and L&S may as well be different schools when it comes to admissions.