i'm no expert but...
My first experience with nikasil was on a 1975 or so BMW Daytona. At almost 100K it still had perfect
oil control when i foolishly gave it away. Most of the BMW aly blocks are coated and unsleeved. It works just fine, with a very low failure rate. It's not cheap, but worth the money and still marginally less cost than sleeving with steel because of increasing labor costs, as opposed to the still-proprietary profit margins of coating.
There are not many down-sides. Break-in is a little longer than ductile iron rings on iron, of course, and a proper ring seat requires modest street rpm's for a while to increase the odds of a quick nice fit. Perhaps not as difficult or improbable as seating chrome rings properly, for instance. A handfull or two of careful dyno pulls will help establish a seal.
Aircraft cylinder jugs (aircooled like the Daytona, remember), respond well to bore coatings. It is a very long story, but now fairly well settled amongst many safety and cost-minded operators that overhauled jugs ought to be coated if the core engine has consumed less than half its useful life between overhauls.
i would agree that coated-bore aly blocks are the way to go, particularly for semi-street engines. i suspect, however, that the proprietary nature of the coating process makes it very important ragarding WHO does the coating and WHAT the history of THEIR coatings is and how financially stable and willing they are to insure warranty liabilities, including components installed in the block.
My bet is that the Kirkham guys will have this all properly covered long before they make any blocks available to the public.
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As daddy used to say:
There is certainly no strategic advantage to wedding an Albatross to a Harrier. The Albatross will be at least disappointed and the Harrier is likely to be very very angry.