My impression is that the solver landscape is a little uneven (the solver reads a Modelica model and computes its solution). For one thing, open-source solvers seem to lag behind the commercial implementations, in terms of capabilities. Even with commercial solvers, there seems to be a non-negligible amount of work around taking correct models that don't run, and reformulating them to let the solvers run them. Caveats -- I don't work on this personally, but know people in my division who do. Also, these impressions are a few years old, and doubtless the situation is improving as time goes on.
[0] Library of component modules for building energy and control systems (http://simulationresearch.lbl.gov/modelica/)
[1] "Spawn of EnergyPlus", a prototype next-gen whole-building energy solver (https://energy.gov/eere/buildings/downloads/spawn-energyplus...)