The raw operation utility functions defined here and here will fail when run inside of a session. Specifically, any function that ends up calling into the getFeatureFlagDoc function will call into getParameter, which will fail the operation with "Cannot run 'getParameter' in a multi-document transaction.". This makes it difficult to convert any time-series tests that perform direct operations on the buckets collection to using the raw operation library, such as txn_ops_allowed_on_buckets_coll.
It's worth mentioning that there might not be any particular reason that getParameter cannot be run within a transaction. As far as I can tell, the command might fail only because it does not override the allowedInTransactions command. There might be some reason why it cannot be run in transactions, however.