I had a Buhk saildrive on a boat that I owned for 14 years. As a broad generality, I found that the saildrive worked fine in general usage. There was noticably less propwalk which took some getting used to and was missed when trying to spin the boat in tight quarters. But for the most part there were few problems in day to day use.
Speaking of saildrives in general, the downside is that aluminum housings tend to be very vulnerable to electrolysis. I coated mine with epoxy when I bought the boat. Saildrives are extremely dependent on proprietary parts and when a manufacturer decides to price these parts like they are made from 'unaffordium', any small repair part such as a zinc, seal replacement, boot leak alarm, or boot replacement can be wildly expensive. Even worse, when a manufacturer decides not to support an older model, the parts become impossible to get.
Saildrive transmissions are miniaturized to fit within the hub of the drive, and so tend to be less robust, and unlike most normal transmissions you cannot routinely buy a rebuilt saildrive.
Unlike a normal marine transmission, you cannot work on the saildrive while the boat is in the water. That includes routine annual maintenance items like changing the transmission fluid and on some models, changing the zincs. It means that you either use the transmission fluid longer than the manufacturer recommends and risk electrolysis or you haul annually whether the boat needs to be hauled or not.
Often there is only one propeller by one manufacturer that will fit the outdrive shaft. If that is a shortlived, unreliable, and badly mfr. supported propeller like a Gori, you were out of luck since that is all that fit on that shaft. Normal props generally do not fit outdrives.
Now then, speaking specifically about the OMC units, these were light and inexpensive adaptations of an outboard motor. They were used because they were cheap to buy and install. If properly maintained, they were reasonably reliable, but OMC's were seen as being short-lived units whose use-by-date has long since passed. That would not be the end of the world if these were easily rebuilt engines like an Atomic 4, but they are not. But the really big (as in deal breaker) issue with rebuilding one is the near total lack of available replacement parts.
To the best of my understanding, OMC stopped supporting these engines a couple decades ago. There were suppliers who had bought up a bunch of left over parts and supported them for quite a while, but last I heard, those folks are no longer around. (Someone please correct me if some company has become the 'Moyer Marine' of OMC saildrives.) So buying a boat with an OMC Saildrive is buying a boat with a somewhat unreliable engine, near the end of its life, that lacks replacement parts, and which is not a 'plug and play' fit with other repowering options. Most of the Cat 27's that I know of that had these engines have had the engine removed, the hole glassed shut, and an outboard on a bracket installed on the transom.
Jeff