Differences among koji hosts

Hello, what are differences among different koji hosts? Are they really the same? I have problem with building MongoDB. In my point of view some build tests are failing depending on the koji host building it. If hosts name starts "buildhw" everything is OK. But with "buildvm" tests are failing... "The testsuite is failing with: going to run test: ThreadedTests::Slack<mongo::SimpleRWLock, mongo::SimpleRWLock::Exclusive>N5mongo12SimpleRWLockE Slack useful work fraction: 0.764695 locks:117504 dbtest: /usr/include/boost/thread/pthread/condition_variable_fwd.hpp:86: boost::condition_variable::~condition_variable(): Assertion `!ret' failed. 2015-06-30T10:59:10.594+0000 F - [testsuite] Got signal: 6 (Aborted)." (see https://bugzilla.redhat.com/show_bug.cgi?id=1237120) Anyone have the same problem? Does someone know where could be the problem? Thanks, Marek