Mongo db is in OpenIndiana and has a portability issues even on Linux
Just few facts:
1) even on Linux MongoDB code has some portability issues and in Debian and Fedora is possible to find some patches necessary to build it on non-x86* HW. Ad the moment Fedora MongoDB has exclude to build on PPC, USparc and s390. In other words portability MongoDB is really not only Solaris issue (again: on Linux)
2) MongoDB as a package is maintained in OpenIndiana and it is few minor issues but despite those small bits everything is OK on OI Solaris.
3) MongoDB uses completely own/custom build framework and i does not uses GNU auto tools, cmake or meson. If some developers choose such custom framework it says many about MongoDB developers.
Conclusion: I would be really holding my tongue about saying anything about Solaris per se in context of MongoDB until those known on Linux issues will be not sorted out first.