A business has to consider its requirements before recruiting new personnel. The restrictive covenants have to be drafted carefully if the business wishes to rely on them.
In relation to onerous non-competition covenants, one way to maximize enforceability is by reference to a geographical area around the business, or by reference to specific competitive businesses.
If the employer terminates in breach of contract by giving short or no notice of termination (called wrongful dismissal), they cannot then rely on the contract to enforce the restrictive covenants. If the employer makes a payment in lieu of notice it will be in breach of contract unless there is an express written term granting the employer a right to pay in lieu of notice.