Paying in cash changes nothing about your legal position. If somebody works for you, you are an employer, and the registrations and deductions apply whether the money moved by EFT or out of a till.
The exposure is bigger than most people expect, because employees' tax is money you were supposed to hold on SARS's behalf. An employer who never registered can be assessed for the PAYE, UIF and SDL that should have been deducted from the day they became an employer, plus penalties and interest, and the business owner can be held personally liable for it. The debt does not sit neatly inside the company.
On the labour side, no payslip means no record, and no record tends to mean the employee's version is accepted at the CCMA. Disputes about hours, overtime, leave owed or whether someone was even employed are far harder to defend without documentation you were legally required to keep anyway.
If you are already in this position, registering late is materially cheaper than being found. The amount owed grows every month it stays unresolved, and voluntary disclosure is treated differently from discovery.