That conversation should take place before you sign something.
After you sign something, it's not important if it's reasonable or not: you have to uphold it.
After you sign something, it's not important if it's reasonable or not: you have to uphold it.
It very much is important, and you don't necessarily have to uphold it. In the vast majority of jurisdictions unreasonable[0] contractual clauses are unenforceable and may even be sanctionable.
[0] what is or is not unreasonable being the primary variable. CNC clauses are generally unreasonable in california (reasonable CNC are related to goodwill and shared businesses) for instance, and in mainland europe they are limited and often must be compensated for their whole duration e.g. in Germany the leaving employee must be paid at least half their gross salary for the whole CNC term (no more than two years) and unreasonable clauses can be invalidated, lest the whole CNC be invalidated.
This is simply not true. Trivial examples include a contract without consideration, or an apartment lease with clauses that attempt to restrict rights granted by local law (very common in NYC). Most non-competes in the state of California are not enforceable as well, even if you agree to it.
That's frankly a ridiculous idea that clearly leads to injustice. Contracts are often between people who are not equals, where one party has less ability to negotiate or walk away. If the weaker party does not have extra protections outside of the contracting process, the stronger party could use its power to abuse them.
This may be your personal moral preference, but it very much is not an accurate reflection of contract law (at least, not in any US jurisdiction.)
You cant sell yourself into slavery for example.