Logical,
Of the different requirements, though, maths is probably the 'least difficult' - the new curriculum has been out for a relatively long time and hasn't changed since. Your Year 6s would have started on it at the beginning of Year 5 at the latest, the sample papers were out in a timely manner, and because ARE won't actually be calculated until after the tests, it is entirely possible that [by dint of a little lowering of the 'pass' mark and application of a normal distribution] the distribution of results will be fairly sensible.
Yes, the requirements of the new curriculum are more challenging, and like you I find there are gaps to fill that will not be there in future years, but on the other hand Maths is not something that requires 'emotional maturity' or 'life experience' or 'wide knowledge of the world' or 'wide reading', so the 'age appropriateness' issue is not one that particularly arises in Maths
The writing, in particular, is much more of a mess. There are only interim assessment arrangements, still. These have changed substantially over the implementation period. Exemplar materials have only just come out, and are 'absolute' standards, not something that can be 'massaged once the final test statistics have come in'. The writing requirements are not age appropriate, as many of the aspects a child has to show will not be familiar to them from age-appropriate reading, and there is no credit given for e.g. being able to tell a good story, interest the reader, create suspense as the assessment criteria are formal and grammatical. Marks for all areas will have to be gained - no 'failing on the symmetry questions but still getting a good mark overall' - EVERYTHING has to be at the same standard to reach ARE in writing.
Might you have a different view if you were an English specialist, do you think?