So a container ship or tanker with a single main engine running on heavy fuel and on transoceanic transits doesn't require as much skill?
Of course using only horsepower as a yardstick to the job facing the engine department of a given vessel is poppycock. It has more to do with being built under inspected vs un-inspected rules being an indication more of the nature and scope of the engineering systems required to be operated and maintained by a given department or engineer.
Would disagree on this point (not that the OSV's are small complex ships part!) precisely because HP should not be the only measure of your experience. But if OSV licenses and DDE licenses are serving an industry segment and there are suitable operational and safety performances being achieved why should they require an unlimited license? On the other hand (and allowing for bad apples in any population) I don't think just about any swinging stillson wrench can move from Chief OSV - regardless of horsepower - to a deep draft, foreign going vessel at the chief level and hope for acceptable performance. NOTE I said JUST about any there are always exceptions. Say for example people who have sailed in the latter trade are familiar with those bigger plants, machinery types, system sizes, may have even advanced their license a bit, etc but then went into OSV's and advanced to CE. That guy is going to have a different outlook on life than a guy who's been told he is mechanically inclined, became an "engineer", likes being called chief and does not really understand the machines he is staring at, pushing the start button for or even disassembling/assembling.
Frankly I don't think many of those latter guys read or participate in this forum though we have all sailed with them in all industry segments.