I think the enforceability of non-compete clauses in employment contracts depends on the degree of restriction and the specific legal jurisdiction. In Singapore, the burden of proof lies with the employer in demonstrating 1). protection required for the employer’s legitimate interests, and 2). restrictions are reasonable in the interests of parties and to the public interest.
Such clauses which restrict competition, or deprive the employee of a job/income would generally be unreasonable and thus unenforceable.