My 6th grader just finished up Delta, but I’m not concerned because she has always loved math & the division concepts (& facts) came easily to her. She got very bogged down with multiplication, mostly memorizing the facts, but surprisingly this year just zipped through memorizing the division facts. I suspect she’ll “catch up” and start Algebra about when she needs to.
My older daughter, however, has been struggling through Algebra in 9th grade & we are finishing up over the summer. She found Pre-Algebra difficult, too. I do recall that she had a much easier time working with the geometry concepts that were introduced in earlier math years, and I’m guessing she’ll find Geometry in high school next year less difficult to master. (That’s my story & I’m sticking to it!)
We like Math-U-See on the whole, and I don’t want to jump from curriculum to curriculum in junior high or high school, so I’m planning to stick with it and just give some extra help if they need it.