Agencies employ most freelancers, be einegstellt for development, should be exclusively your own programmers to use. The focus is on a fast and efficient programming of the project. This manifests itself in a poor code quality, d.h. practical, If an other programmer to change something later, then it takes ages, because the code is so bad, that you can't see through. Good code is especially readable and understandable, what it does need experienced programmers, which given also time is to write good code.
Agencies, the programmer will not in any way stop to write good code and often you choose as freelancers only “in exceptional cases” to work with an agency, because this usually with many overtime, Time pressure and low pay goes hand in hand. That's why land not good programmers at agencies, but rather the black sheep.
My recommendation is, not to put in the development of software on an agency, but rather to one or several good Freelancer, which implements the project conscientiously and in direct communication.
The comparison of offers by various freelancers should be not only on the price, but a focus on the experience of the programmer and the quality of the resulting software.
I have collected my thoughts about it from my and the experiences of other freelancers over the years in Berlin.