I owned a 1986 Newport 28 from 2004 to 2007. I enjoyed the boat very much. I can't comment on Islander boats, so I can't help you there.
A few things for you to know:
Capital Yachts, the maker of Newport, Gulf, and Neptune yachts, went out of business in the early 90's.
There are no design problems in the Newport 28 hull to deck joint. That doesn't mean that any particular boat is OK, just that the design and manufacture of the hull / deck joint is in line with other boat builders.
As a data point for you, I sold my boat for $14K. It had a brand new sails (1 year old UK dacron main, 1 year old UK tape drive laminate roller furling 140 genoa), good diesel engine, and was in good condition. It had a good autopilot, Lowrance GPS / SONAR, and a number of other things including cruising spinnaker in dousing sock.
I enjoyed the Newport. It was a little tender, but sailed well. Compared to other boats, like Catalina and O'day, the interior was nicer, with more teak, and nicer cabinetry. The boat came standard with self tailing winches, nice traveler, and lines led aft.
Bad things were the location of the ice box thru hull, leaks around the chainplates (common to many boats), and small head (what do you expect on a 28').
You can learn more about Newport boats here:
Newport, Neptune, Gulf sailboats built by Capital Yachts