When developing based on fixed price development, I made sure that this approach allows you to clearly control the budget and deadlines. On the one hand, this is convenient, because everything is agreed in advance, and I know how much this or that stage of the project will cost. However, you need to plan and describe the tasks carefully, because any changes may require additional payment. In Magento 2 development, this approach is especially useful if you have a limited budget or tight deadlines. It is important to choose trusted developers so that the result meets expectations.