At the time of writing of this ticket, there are 38 sharding passthrough suites and only 24 of them are initializing more than 1 shard (by default num_shard=1).

$ git grep ShardedClusterFixture buildscripts/resmokeconfig/ | grep passthrough | cut -d':' -f1 | wc -l
38

$ for suite in `git grep ShardedClusterFixture buildscripts/resmokeconfig/ | grep passthrough | cut -d':' -f1`; do grep num_shards $suite; done | wc -l
24

This means that plenty of tests are being executed on a one-shard cluster whose behavior is pretty much similar to a plain replicaset, without spotting potential errors happening in multi-shard environments.

As an example of drawback, the jscore passthrough suite is only initializing one shard and did not catch a severe bug (SERVER-66031) introduced by SERVER-62455. Simply increasing the number of shards would have resulted in consistently hitting the problem.