Just weighing in on this because I have a) a little time and b) an opinion (I know; everyone has one and most of them ...)
This type of compensation structure might not be strictly for the employer's benefit. Depending on the employee's situation and other factors related to the employment, it could benefit both. Let me explain.
First, it is very unlikely that the employer is using this to pay less than minimum wage to the employee. It is far more likely that they are paying above minimum wage, even on the stated salary. The stated salary is less than total compensation, but probably not less than minimum wage so the tax burden is reduced. And, assuming that the tax burden would be shared by employee and employer, then the reduction benefits both.
From the standpoint of healthcare, there is likely no difference between paying Seguro Social on $5,000 of income per month or on $15,000; the facilities, doctors, medicines, etc. will all be the same. The only real difference I can see would be in retirement. Someone declaring and paying Seguro Social on $5,000 per month will obviously have less in retirement than someone paying taxes on $15,000. However, this would only matter to someone who is planning to retire in Mexico based on what Seguro Social pays in retirement.
I have no clue as to the legality of such arrangements in Mexico. However, I can see instances where the benefits to such arrangements would accrue to both employer and employee.