They are as friendly as the law makes them, like any other company.

Note also that although France is often cited it actually has a very low unionisation rate, lower than the US, and e.g. the UK is massively more unionised. Unions are mostly active in the public sector. A specificity is that the right to strike is a constitutional right with quite lax requirements and it is a recurring debate that it is abused by unions. Union activity is also extremely protected so if you get elected as union rep it is virtually impossible to fire you for any reason.