Devstack works. Sort of. Poorly. Everything else is pay-a-consultant territory.
OpenStack refers to a number of different projects (Nova, Neutron, Horizon, etc). I would advise getting a high level understanding of how Nova, Neutron, and Cinder/Swift interoperate. Then drill in to Neutron. As with every other cloud platform I've worked with, most of the IaaS complexity lies in Networking, in my opinion. Neutron, and SDN in general, is a fairly complex topic depending on your experience with network architecture and administration.
So for example a development environment I use is a docker-compose environment building Monasca. That environment is a work in progress, so half the time I'm fighting that environment just to make a proper change.
Then from there, I don't have Horizon, so I have to use command line apis directly to set everything that I wish to use.
The whole thing has just been a challenge, every part I've had to learn in some respect. The actual code I've changed in Monasca, has been the simplest part so far. Validating it's correct, and testing it has been the far harder challenge.
Monasca also being a bit of a challenge since it's several different repos of services working together, instead of a large one akin to Nova.
I'll get it, but it's been a struggle so far.