Hard to say if related to Mongo or to MongoRocks specifically, but I had both WT and MMAPv1 nodes connected to the same replicaSet, and they remained healthy.
2015-12-07T10:07:28.136+0000 I COMMAND [repl writer worker 11] dropDatabase Tenant_1000 starting
2015-12-07T10:07:28.151+0000 I - [repl writer worker 11] Invariant failure entry->getTotalIndexCount(opCtx) == entry->getCompletedIndexCount(opCtx) src/mongo/db/storage/kv/kv_database_catalog_entry.cpp 323
2015-12-07T10:07:28.184+0000 I CONTROL [repl writer worker 11]
0x121d63e 0x11be10e 0x11a0662 0xef92ca 0xefd557 0x9c2754 0xafebd5 0xaf567a 0xaf688e 0xaf781c 0xe0d94c 0xe8c000 0xe91875 0x11b3505 0x12710bd 0x7f4858b51182 0x7f4857c4847d
----- BEGIN BACKTRACE -----
{"backtrace":[{"b":"400000","o":"E1D63E"},{"b":"400000","o":"DBE10E"},{"b":"400000","o":"DA0662"},{"b":"400000","o":"AF92CA"},{"b":"400000","o":"AFD557"},{"b":"400000","o":"5C2754"},{"b":"400000","o":"6FEBD5"},{"b":"400000","o":"6F567A"},{"b":"400000","o":"6F688E"},{"b":"400000","o":"6F781C"},{"b":"400000","o":"A0D94C"},{"b":"400000","o":"A8C000"},{"b":"400000","o":"A91875"},{"b":"400000","o":"DB3505"},{"b":"400000","o":"E710BD"},{"b":"7F4858B49000","o":"8182"},{"b":"7F4857B4E000","o":"FA47D"}],"processInfo":{ "mongodbVersion" : "3.0.7", "gitVersion" : "863349f8cafae87c1ab6fefdf2781107403181c5", "uname" : { "sysname" : "Linux", "release" : "3.13.0-52-generic", "version" : "#86-Ubuntu SMP Mon May 4 04:32:59 UTC 2015", "machine" : "x86_64" }, "somap" : [ { "elfType" : 2, "b" : "400000", "buildId" : "107E079760C1CD46B0AFFA2E9F8E22C45929D523" }, { "b" : "7FFDA3A0E000", "elfType" : 3, "buildId" : "F8E01F1BCB9F27FF04FC2B8C9454AD8EACB7B37C" }, { "b" : "7F4858D67000", "path" : "/lib/x86_64-linux-gnu/libbz2.so.1.0", "elfType" : 3, "buildId" : "E1031DDBFFE20367E874B7093EEC0C8D9F3B43F6" }, { "b" : "7F4858B49000", "path" : "/lib/x86_64-linux-gnu/libpthread.so.0", "elfType" : 3, "buildId" : "9318E8AF0BFBE444731BB0461202EF57F7C39542" }, { "b" : "7F4858941000", "path" : "/lib/x86_64-linux-gnu/librt.so.1", "elfType" : 3, "buildId" : "92FCF41EFE012D6186E31A59AD05BDBB487769AB" }, { "b" : "7F485873D000", "path" : "/lib/x86_64-linux-gnu/libdl.so.2", "elfType" : 3, "buildId" : "C1AE4CB7195D337A77A3C689051DABAA3980CA0C" }, { "b" : "7F4858430000", "path" : "/usr/lib/x86_64-linux-gnu/libstdc++.so.6", "elfType" : 3, "buildId" : "E21BCC61BDD4F9040CAE8DAC93E0B5AF94C61E16" }, { "b" : "7F485812A000", "path" : "/lib/x86_64-linux-gnu/libm.so.6", "elfType" : 3, "buildId" : "1D76B71E905CB867B27CEF230FCB20F01A3178F5" }, { "b" : "7F4857F13000", "path" : "/lib/x86_64-linux-gnu/libgcc_s.so.1", "elfType" : 3, "buildId" : "943D68EB5F75B05C52106573478422B3685D3BFB" }, { "b" : "7F4857B4E000", "path" : "/lib/x86_64-linux-gnu/libc.so.6", "elfType" : 3, "buildId" : "30C94DC66A1FE95180C3D68D2B89E576D5AE213C" }, { "b" : "7F4858F77000", "path" : "/lib64/ld-linux-x86-64.so.2", "elfType" : 3, "buildId" : "9F00581AB3C73E3AEA35995A0C50D24D59A01D47" } ] }}
mongod(_ZN5mongo15printStackTraceERSo+0x3E) [0x121d63e]
mongod(_ZN5mongo10logContextEPKc+0xEE) [0x11be10e]
mongod(_ZN5mongo15invariantFailedEPKcS1_j+0xC2) [0x11a0662]
mongod(_ZN5mongo22KVDatabaseCatalogEntry14dropCollectionEPNS_16OperationContextERKNS_10StringDataE+0x53A) [0xef92ca]
mongod(_ZN5mongo15KVStorageEngine12dropDatabaseEPNS_16OperationContextERKNS_10StringDataE+0x2B7) [0xefd557]
mongod(_ZN5mongo12dropDatabaseEPNS_16OperationContextEPNS_8DatabaseE+0x144) [0x9c2754]
mongod(_ZN5mongo15CmdDropDatabase3runEPNS_16OperationContextERKSsRNS_7BSONObjEiRSsRNS_14BSONObjBuilderEb+0x305) [0xafebd5]
mongod(_ZN5mongo12_execCommandEPNS_16OperationContextEPNS_7CommandERKSsRNS_7BSONObjEiRSsRNS_14BSONObjBuilderEb+0x3A) [0xaf567a]
mongod(_ZN5mongo7Command11execCommandEPNS_16OperationContextEPS0_iPKcRNS_7BSONObjERNS_14BSONObjBuilderEb+0xF7E) [0xaf688e]
mongod(_ZN5mongo12_runCommandsEPNS_16OperationContextEPKcRNS_7BSONObjERNS_11_BufBuilderINS_16TrivialAllocatorEEERNS_14BSONObjBuilderEbi+0x66C) [0xaf781c]
mongod(_ZN5mongo4repl21applyOperation_inlockEPNS_16OperationContextEPNS_8DatabaseERKNS_7BSONObjEbb+0x62C) [0xe0d94c]
mongod(_ZN5mongo4repl8SyncTail9syncApplyEPNS_16OperationContextERKNS_7BSONObjEb+0x320) [0xe8c000]
mongod(_ZN5mongo4repl14multiSyncApplyERKSt6vectorINS_7BSONObjESaIS2_EEPNS0_8SyncTailE+0x85) [0xe91875]
mongod(_ZN5mongo10threadpool6Worker4loopERKSs+0x3E5) [0x11b3505]
mongod(+0xE710BD) [0x12710bd]
libpthread.so.0(+0x8182) [0x7f4858b51182]
libc.so.6(clone+0x6D) [0x7f4857c4847d]
----- END BACKTRACE -----
2015-12-07T10:07:28.184+0000 I - [repl writer worker 11]
***aborting after invariant() failure