The 888 was the last of the 851/888 series and the most developed too.
On my 888 SP5 only the charging system gave me problems - this is a weakness tha carried right through to the 996.
I'm not aware of any endemic issue as they were pretty well sorted, however I have a friend with an 851 (from around 1990), it's tight but has been known to have electrical issues which have manifested themselves in poor running. These problems are directly related to age and the consequential corrosion in the electrical connections.
So if you buy it I'd suggest you get some diletric grease, find all connectors and fill them with the grease.
I agree with the last post. I sold mine off several years ago and with the exception of a few wiring issues (it IS Italian, after all) the bike ran pretty well. Nice looking bikes and fun to ride. Good luck. Dave