The class was certainly hard; it was much, much harder than the grade distribution would imply. Note that standards were relaxed during the COVID quarters (Spring 2020 and 2021), and tons of people Pass/No Passed this class. That said, I have no complaints about Srivastava himself; most complaints would be about course logistics more than anything. Lectures were good and he clearly cared about student learning.
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We had a flipped classroom model due to COVID, where we watched recordings of the lectures and then came in during lecture to ask questions and go over the material. The recordings were excellent, and they came with excellent lecture notes. I found they were good enough that there was no need to read the textbook at all. Lectures, weekly review sessions, and discussions were all optional and recorded, so international students weren't penalized at all. Only issues I have on the discussion end would be real-life applications. The class felt far too theoretical.
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The class was definitely harder than the previous classes in the series, especially compared to Dr. Eisler's classes. Part of this was due to the fundamental nature of the class; it builds upon 101A, which was definitely not easy. A bigger factor would be logistics. Homework was worth a whopping 30% of the grade, and it was NOT easy. Many people got below 70% on them. Quizzes, worth 10% of the grade were also tricky despite being multiple-choice, with similar averages compared to the homework. Homework was somewhat on the level of exams. Unfortunately, this meant that the exams, instead of taking the intended 2-3 hours to do, actually took far longer. Had we not been given 24 hour exam windows, most students would've been crushed on the exams. Compounding difficulties with logistics was the fact that it was hard to determine where we lost points on exams (all the TAs did was assign point values for problems).
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Finally, like the previous reviewer said, reviewing velocity profiles would be very helpful for the course.

Ch Engr 101C with Srivastava was difficult, but not in the way you might expect. I didn't think that the class was more conceptually difficult than, say, heat transfer or momentum transfer, but there was a huge emphasis on math. For example, you might be asked to verify a certain concentration profile, and the math can get pretty tedious as you solved for the integration constants. (I had to use the quadratic formula once to get one integration constant). And the math only becomes worse as you delve into PDEs, which you need to solve through similarity method or separation of variables. Many of us did not have a good foundation with error functions and gamma functions, so it got pretty confusing at times. However, Srivastava was pretty clear in his lectures and with his expectations for the class. I expected to get a B+ in his class because he said that 1 standard deviation above was A- and 2 standard deviations above was A. I was like there's no way I can score 20 points above average on every single exam. But it appears that he lowered the threshold because we did pretty well overall as a class.

I don't really have many complaints... course material is difficult, yeah, and math-heavy, but I honestly hardly know a thing about the more obscure math we go over in class (similarity methods, error functions, laplace transforms, etc) and I still did OK. Obviously, you'd want to have a decent grasp of those concepts to snag an A, but I would say this was one of my laziest quarters but this class was still manageable.
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This was UCLA's first fully Zoom quarter, so there were a lot of quality of life resources for us to study at our own pace. Sam gave you ALL the class resources... as a result, I watched lectures once in a while when I needed elaboration on a slide he wrote (all slides are constructed during lecture and posted on CCLE immediately after). Still, from what little I've seen, Sam is really clear and sensical in his presentation of course material. He puts thought into how he teaches. Wow! I wouldn't call Sam the best professor ever, but he's definitely a step above average. He's what you expect from a professor that gives a shit. Don't know what he did to be called "terrible." One thing about the class that did blow chodes is the inclusion of required readings and NOTES for those readings... notes that are available for the entire class to read. I found that often made the readings a race to read first, or you're in danger of just repeating what other students have said. You get graded on the quality of your comments too... that was definitely the shittiest part of the class IMO.
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Overall, if you've made it through A and B, 101C is just a rehash of those two classes. Dimensionless numbers, deciding boundary conditions for some conservation equations, etc etc. The most unique stuff to this course is probably the concept of a mass transfer rate and using that to determine the time some mass A needs to completely diffuse through some medium (don't remember stuff like that in A and B... maybe in B as heat transfer rate). A and B gloss over some difficult math, but I would agree that Sam expected pretty solid math skills on his exams. Albeit, he typically provided math "hints" for those problems, but solving a quadratic equation where a = (phi^2 + .56*phi*gamma-gamma^3) and b and c are just as ugly during an exam... that sucked. If he lightened up the math expectations a bit, course would be 100% fine.
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A final tip: brush up just a little bit on your velocity profile derivations for the second half of this course. It'll come in handy!