My high school student did Alpha through Algebra I before transitioning into 8th grade public school.
He "passed" Algebra I on his placement test, but it was close. Due to MUS's reputation for being "light" the school recommended we put him in pre-Algebra for 8th grade. A few weeks in we realized that he had the concepts down just fine, it was the terminology that caused him trouble on the placement exam. Mr. Demme's terminology didn't always align with 'standard' terminology. Or my son just forgot the terms. Probably both. He was bumped up for the second quarter when he was found tutoring the Alg 1 kids at lunch.
This student returned home for high school, and dual-enrolled in math classes at our local state u. On his own, he requested to continue with the MUS sequence. So for Geometry, Alg 2, and Pre-Calc he did the fall semester at college and then the spring semester at home working through MUS. He took the MUS tests, and for any concept he earned less than 90 he completed the lesson. Again, he decided and did this on his own.
He is not a mathy kid. He used to fight MUS ... crying, screaming, outright refusal, even cheating by sneaking the TM. Once he got to 8th grade public school, he realized MUS wasn't so bad. He says public school taught him math shortcuts, but MUS taught him the how/why. He's a kid who needs that. Not all do. He took the PSAT three times, never with a calculator. His lowest math score was 710. His math grades at college were "B"s, so it's not like he was a math genius.
I had planned to drop MUS for Lials. I had concerns about MUS in high school. This son requested dual enrollment so we went with that instead. Our journey back to MUS surprised both of us. I'm still unsure if I trust it as a standalone for high school, but I'm glad I didn't discard it entirely. I'm holding onto the high school levels to use with my younger kids, even if only as a supplement.