Our experience is that for a child who finds ALEKS motivating, it can become a problem to let them get too far out of the level where they are working at school. Our DS was 5 years/grades out of sync with age peers; we recently pulled the plug and stopped giving him ALEKS math, but in the meantime there is no sensible class to put him into, because he knows most (but not all) of what's covered in several different grades.
I think it's fine as a supplement, but it's not a replacement for math instruction, and I've seen a real downside in letting a kid work as far ahead as he can.
I think there are two different potential problems, and it's worth separating them.
a) ALEKS questions are very formulaic. It's quite possible for a child to do well at a certain level in ALEKS and yet be completely flummoxed by a differently posed question using the same material. This hasn't been a problem for us as DS is very keen on maths (and so) is doing lots of other maths, insists on understanding everything, and is motivated more by learning cool maths than by completing his pie, but if all these elements weren't in place I think it could be a major problem.
b) Allowing your child to get ahead of their age peers creates a problem for class placement. If you have a decent alternative - if going broad rather than ahead is feasible, e.g. if you can satisfy your child's needs using problem-solving based on grade level material rather than advancing through the grades - you probably want to take it. In our case, it was clear very early that class placement for maths was going to be a problem whatever we did. Once you're already in a situation where there's no suitable class for your child and they need completely special treatment, there's nothing more to be lost by letting them get further ahead, and you can ignore this aspect. This is as much personality and school attitude as maths - DS took the first two years of school to complete the maths syllabuses offered by his school, and at the end of the first year, when he was several years ahead but still within the school's syllabus, we did discuss putting him in a higher class. He didn't have the social and emotional maturity for that to have worked then, though, and the opportunity passed quickly.
