[susan53]

Thank you for the Xmas greetings - just back from a lovely holiday in Germany with family.

If something is "open to criticism" it simply means that it can be criticised - ie there are obvious weak points.

Here are some other examples from various corpora :

However, despite the insight of many of his observations, his own conclusions are open to suspicion because of his failure to employ at all times the correct research methods.

it is still used in making current population estimates in post-census years, though the value of these estimates is open to question.

...but in the absence of textual evidence this is necessarily speculation based on archaeological artefacts and open to debate.

Relating this discussion back to the labour theory of value is problematic as Marx's definitions are open to interpretation and it is therefore impossible to provide definite categories of productive and unproductive labour.