The file format specification allows you to easily construct a writer or your application that write the output exactly as you want it to. That is the big advantage of the vtkhdf compared to other file formats VTK use. I have for instance a parallel writer in Fortran that write time-dependent PolyData from my MPI application into a single vtkhdf-file. That was quite straight-forward to implement from the file format specification.
In my opinion the most important is that the file format itself is specified in a way that allow efficient read and write, independent on the actual implementation. Implementation details can always easily be changed, but the specification is more or less carved in stone once it is out. As far as I see it that is the case.
Then the second most important thing is that the reader is performant and reads the vtkhdf-files in an efficient manner. As far as I see it, that is also the case here.
Then, the official writer is a least priority here… Not that it is not important, but I think it is a lot of work to make the writer fit every HPC application and file system combo in the universe.