It is notable that only 48 UN Member states have ratified the convention (More help and treats for foreign workers) - and they are countries from which migrants come (rather than those to which migrants go). East Timor, Egypt, the Philippines and Senegal are some of them.
This suggests that it is detrimental for countries which expect to receive migrants to sign it.
There are many problematic aspects of this convention (Text: International Convention on the Protection of the Rights of All Migrant Workers and Members of Their Families). I shall just look at Articles 10, 29 and 43.
Article 10 says that no migrant worker or his family "shall be subjected to torture or to cruel, inhuman or degrading treatment or punishment". This has the potential to subvert a country's legal system. For example some states in the US practise capital punishment, which some argue is cruel and unusual punishment (The Case Against the Death Penalty). Ratifying this convention might mean that migrant workers and their families could not be subject to capital punishment. This might cause a legal crisis in countries practising capital punishment.
Article 29 says that "each child of a migrant worker shall have the right to a name, to registration of birth and to a nationality". The last seems the most problematic. In countries which don't practise jus soli, or the right of those born in a country to be citizens (i.e. most of the world), you need to have at least one parent who is a citizen of the country in question. Now, what happens if the child of a migrant worker is unable to obtain the nationality of one or both of his parents' countries? A country signing the convention would be obligated to give this child citizenship of its own country.
Article 43 calls for migrant workers to have equal access to nationals with regard to housing, including social housing. Social housing is typically subsidised or otherwise priced below market prices, and countries understandably would prefer for their citizens to benefit from this first before opening it to migrant workers (if at all).