There is a very high risk involved in determining the value of selling a part of a business. If the entity that is being sold or acquired is not on its own financial accounting system, defining exactly what is being divested and then valuing it accurately requires a very deep dive into the operations and finances of the parent company during the due diligence process. Even then, the true nature of exactly what pieces are being sold (the child company) and their profitability are estimates that are often ascertained later and are subject to either an upward or downward conditional post-closing price adjustment. The price adjustment mechanism is a hedge against incorrect beliefs about value when the full details are not known, but it is a blunt tool that can often result in over- or under-payments of millions of dollars or more than if the deal had been predicated on accurate descriptions of what is being divested and the historic financial performance of the spin-off part of the company.
There is a risk for the parent company in not setting the correct price, and for the acquiring company of paying too much.