I haven't had a failure of a block. A friend had some failures but due to wrong size.
However I have failures with Harken cam cleats (broken spring) (downhaul). And I am very dissatisfied with my Harken mainsheet. The cleat is only adjustabe by three settings. The angle between is far to large. I am also totally disapointed by my Harken traveller, which despite extra care and maintenance has far too much friction. I am probably spoiled by the Dart 18 traveller mainsheet combo. Something that simply worked without any mainetencae after 13 years of use in saltwater and storage on the beach.
For those who don't know. The Dart traveller is from I.Y.E.. We had the same system on the T, which still worked after over 30 years. The reason is very simple, it uses wheels instead of small balls.
I have also some parts from Ronstan which are permantely exposed to sunlight (outhaul, spin halyard) which still look as new.

Cheers,

Klaus