Unlike some of the simple and hosted alternatives, users need to host and setup Jenkins by themselves. This results in both a high initial setup time as well as time sunk into maintenance over a projects duration.
Debugging on remote build agents is a nightmare (especially without isolated builds). Concourse CI can be run locally. When there are problems with the pipeline definition, it can be run and debugged locally. That means it takes less time to find and fix problems.
The downside of building on BOSH is that a full, scalable deployment of Concourse CI requires AWS, vSphere, or OpenStack. If you don't already have these, any one of them can be a big effort to set up, just to get a build server running. Might not be a good fit for smaller teams.