Although Koa is a great choice for building HTTP services and web applications, it is not always the best option for all projects. Just like with every framework, language, and even design pattern, the very things that are advantages become drawbacks when dealing with certain use cases. It is best to judge the use of Koa on a case by case basis and make use of it only when it is a good fit for the particular project.
Generally, Koa will not be a great choice for you if you are not willing to try out the newer JavaScript ES6 syntax. Koa was built for the modern web. If, for some reason, your project has a strict requirement to use an older version of JavaScript, Koa would not be suitable.
If you would also prefer a framework with a lot of boilerplate code and a defined structure, you might have to look at other frameworks such as hapi.js and AdonisJs. Koa, much like Express, prides itself on minimalism and allowing developers to be expressive. Having a lot of boilerplate and a strict code structure are not philosophies Koa embraces.
It is also important to note that Koa is a framework built on top of JavaScript and Node.js. As Koa inherits the advantages of Node, such as being fast at performing network and asynchronous operations, it also inherits some of the drawbacks and limitations that are present in Node. Koa would not be an ideal choice for a project where JavaScript is not the language of choice.
