Do you think 40 hours a week is bad for a work-life-balance?
No.
Are there jobs they are fairly underpaid in terms of the requirements in your opinion?
Yes.
I believe that some healthcare jobs, a few very sensitive roles that require a highly specialised skillset and regular antisocial hours shifts are underpaid. Most should be tax free or eligible for free/low cost accomodation nearby to workplace venue.
What do you think is a fair salary a month, independent on the job and the residence?
It depends on a variety of factors.
The local region, type of job, how dangerous the work normally is, level of qualifications & experience required, average hours per day/working week, average expected weekly/monthly overtime and if it involves any all nighters or working antisocial hours shifts with little notice.