Hi Scott Lee,
Did you made any progress on this topic? We are also looking into Mendix and I have a similar question: Should we go for Mendix only for processes design and execution or use an external engine like Camunda.
Thanks in advance for an update.
Hello Scott Lee,
We @ Tiruven have extensive experience of BPM Suites/ Workflow tools, commercial (Pega, IBM, Opentext, Appian, Bizagi) as well as open source (jBPM, Activiti, Camunda).
Of late, we have started to use Mendix as strong alternative for automating processes/ workflows, which of course calls for building  reusable process/ task management frameworks in Mendix as well as  process anaytics modules.
We have also dabbled with the idea of embedding light-weight process engines, like Camunda, in Mendix apps and using Mendix for UI/ Entity/ Integration etc.
There are multiple factors that determine adoption of appropriate solution pattern and development platform.
What we have also realized from our experience is that knowledge of BPM Suites really helps in building the right design to automate processes/ workflows in Mendix.
Please connect with me at firstname.lastname@example.org to discuss in detail.
We would be very eager to demonstrate our thought leadership & Mendix implementation on this.