Some clients have a rather relaxed perception of what software engineers do and as such it increases their risk tolerance with software solutions, what I mean is that some people believe that because software engineers can easily release patches and updates over the cloud they (the client) don't have to wait for the project to be fully done before launching it.
Hurrying a software engineer can be really dangerous as they may end up skipping unit testing, exception handling and other necessities that must be taken into account in order to release a dependable solution. Sure, an update or a patch can be deployed, the more important question you need to ask your client is how much loss are you willing to sustain before said patch is released?
Clients due to their insufficient knowledge of software programming tend to underestimate how bad things can get, you'll take the blame if you release very terrible code just to make some poorly conceived deadline, you need to be firm or at least have a legal document protecting you from the consequences of a premature software solution.