Within finance, I've worked more on building trading application, while oneZero builds the infrastructure. For me, it's more exciting to work on trading systems, but that's personal preference.
Some parts of the code base are getting a bit unwieldy, in line with normally happens for a project of this age. This is mitigated by many long-term employees still working there and being more than willing to help you navigate some of the denser areas. Also, I was given the opportunity to work on several sizeable projects where I got to write mostly new code.