Do you already have the seat? It wouldn't be one I'd recommend if you're looking to purchase, although I disagree with the idea that they are "really dangerous" - they do pass the legal crash tests which is something, and they are safe as long as they are used properly. If you've already got one, and it currently fits your child and car well, then it's fine to use it. But it is true that any seat that is trying to do so many jobs at once is likely making compromises in some or all of them, and seats tailored to each stage do generally do a better job in terms of safety, comfort and ease of use.
I'm not sure how far this one expands. It is a common issue with these multi mode type seats, that the booster mode doesn't last nearly long enough for older children. However, there's another common issue (and I apologise I can't make sense of the manual to see if this is the case) that with seats which have both a booster mode and 5 point harness mode, there is a maximum position for the headrest with the harness attached, and then once the harness is detached or stowed away, you can raise the headrest up further. This page shows a picture of it in the highest headrest position, so I would check if yours goes up as high as this and if it still seems too small.
https://www.pushchairexpert.com/advice-inspiration/reviews/cosatto-all-in-all-review/
I have found a couple of pictures of children in Cosatto all-stage seats (not sure if they are the exact model All in All) who look as though they could be around 7-8 years old. This does look close to if not the higher limit of the seat, so it might be that you'll struggle to get a decent fit in the seat at the upper end of the age range.
You would also want to check whether the seat offers a good lap belt fit, as again this is a common issue with multi-stage car seats. A booster seat should route the belt over the child's hips or the top of their thighs, touching the hips. If it's over their tummy then it's too high and it's unlikely to offer good protection in a crash. I can't see the belt well enough in these pictures to say if it does or not.