There are several limitations with db.currentOp(), including the documented 16MB result set size. There have been various testing issues because of this, such as SERVER-47513 where the command gets truncated due to the size of the response.
is documented by
DOCS-14290Investigate changes in SERVER-47603: Rewrite db.currentOp() shell helper in terms of $currentOp aggregation stage
is related to
SERVER-51463currentOp command silently fails to apply match conditions when result is truncated
SERVER-55175Maintain original read concern for $currentOp in passthrough suites that change read concern
SERVER-55174Investigate test failure in sharded_collections_causally_consistent_jscore_txns_passthrough suite with db.currentOp change
SERVER-55603Shell read preference behavior is wrong for currentOp