It is a cautious call any product owner has to take depending on the state of the product.
- If you are trying to build the prototype of the product with AngularJS to show it to your early adopters and get the feedback then you can outsource the complete development to an agency and get the work done. As the agency will have complete control over the prototype and build it from scratch. Usually, startup founders follow this approach to analyse product-market fit.
- If you already have a product and the frontend framework with which your app is being built is AngularJS then it is better to hire an in-house or a dedicated developer from an agency(In case you want to start quick and save some bucks on development cost).
- If you are starting from the scratch (You have researched and understood that there is a market fit for your product) and you have decided what stack to use to build your product then you can choose to outsource complete product development to an agency which posses the required skill set for obvious advantages of outsourcing.
AngularJS is a frontend framework and in most of the cases, you must have backend API to complete a web app. So outsourcing only AngularJS work is always a hassle and finding a dedicated developer from an agency is a good option.