Update 16 July 2021 the CJEU yesterday held along the exact same lines.
In Joined Cases C‑152/20 and C‑218/20 Gruber Logistics and Samidani Trans (my shortening of the many parties involved, Advocate-General Sanchez-Bordona opined yesterday (no English version available at the time of writing).
The Opinion showcases a number of complex levels in Article 8 Rome I, the protective regime for individual employment contracts. The case also points to the complex task in addressing social dumping in the EU. The two cases involve a classic case of such dumping, namely international freight transport.
The AG first of all reminds the referring judges that they must consider whether Directive 96/71’s provisions on minimum wage for posted workers might not be applicable in the case (the referral decisions suggest they are not and the issue is not part of the preliminary reference).
He then dissects the cascade of Article 8 which, similarly to consumer contracts, gives parties full autonomy for choice of law with however a correction for the mandatory provisions of the default law which would apply if no choice of law is made. Whether provisions are mandatory or not, including for minimum wage and despite CJEU support for them being mandatory (Sähköalojen ammattiliitto, Case C‑396/13) continues to be the subject of national assessment: there is no EU harmonisation on same.
As for whether employees have truly consented, the odd provision of Article 3(5) Rome I means that it is the putative law which determines consent (this is notably different for the issue of consent for choice of court under Article 25 Brussels Ia). He does suggest that the Romanian statute at issue in one of the cases, should it (an issue left for the referring judge to decide) in fact oblige employees to consent to choice of law for Romanian law, negates true consent.
European Private International Law, 3rd ed. 2021, 3.36 ff.