The math snob in me turned my nose up at certain programs for YEARS because if their non-challenging, drill-and-kill, etc. reputation. And then out of desperation during pre-algebra, I took my oldest to a homeschool store and told him to pick whatever one he wanted. He chose Saxon. At first I was embarrassed to even tell people we were using it. LOL!! It has made SUCH a difference in his speed, confidence, and overall preparedness for algebra! I had written off the constant daily review as "too much", but now I can see that it was exactly what he needed to internalize all of the things needed before he jumped into algebra.
My third son is good at math, but he has 2 older brothers who greatly excel in math, and one day he told me he was "dumb at math" because he was struggling with something in the Singapore 2nd grade book. After talking with a respected friend, she told me how her son who was frustrated in another program really blossomed with Teaching Textbooks, so I gave my DS the placement test for the 3rd grade book, and he did fine. He is SO PROUD of the fact that he is doing THIRD grade math even though he is only in 2nd grade, and he is getting 20-21 correct out of 22 every day. It isn't necessarily that the program is so much simpler, it just explains it in a way that he totally gets, whereas he just didn't with Singapore. Yes, I agree that the scope and sequence is not as advanced in Teaching Textbooks as other programs... but that is not always a bad thing. He is still learning all of the things he needs to know. Not everything that he COULD know, but everything that he NEEDS to know. And... he is motivated to get to 4th grade math before the end of 2nd grade (both of his brothers did 4th grade math when they were in 2nd, but with different programs), so many days he CHOOSES to do a second lesson, and since the lessons are 100% independent, it is totally fine with me (doing 2 lessons per day in another program would NOT work for 5 kids LOL).
A long way of saying... don't base a decision this important on what does or does not work for someone else. If the level 5 Singapore books are that frustrating, it's most likely because something isn't solidified from one of the lower levels. Switching now (but sticking with a program, not continuously jumping ship when it gets hard) seems like it will bring a peace to your school day that is needed. Saxon and Teaching Textbooks are both really solid programs, it just took me a few years to admit it. :) MUS isn't bad either, but you would have to add things to it to beef up the word problems and fill up the time (just my personal opinion, of course, but my son was spending about 40 minutes PER WEEK when we tried MUS, and that just isn't enough time spent in math mode).