It seems recent tickets are restricting use of capped collections with transactions, e.g. SERVER-47824. However, it seems that if a user knows what he/she is doing, they should have the option to enable transactions with capped collections for both read and write. Perhaps such option could be disabled by default and required user to set a param on the transaction (allowCappedCollections: true).

My use case: I use a lock collections for my transactions, and upsert documents to it with the scope I want to lock. This lock collection grows indefinitely, and so setting some reasonable size cap (100MB) would be ideal. In practice the locks I would ever write at any one time would never be more than 1MB even in a degenerate scenario, so there is no way the cap is going to affect the transactional integrity of my system in the real world.