Small, but I suspect significant (in contrast to, say, the vanishingly small number of keel failures). On our year transat sailing we knew of two or three steering or rudder failures (that is, boats in the vicinity at the time, or people who knew people we knew..) - including a family who arrived in Madeira while we were there who had a failure a long way offshore and endured a long-distance tow at high speed behind a container ship.. not fun. I don't recall if this was a spade rudder, but given that steering failure (i) is really not that rare and (ii) has a very high chance of ruining your day if you sail out of reach of assistance, I would say prioritising a design with inherent strength in that area is not a bad move.There are thousands of spade rudders in use like my friends Dufour that was lost in the Atlantic when the rudder was bent backwards and jammed hard against the hull. Major manufacturers are still constructing rudders in the same way as far as I know. Whilst in the Azores this summer on my way back across the Atlantic two Jeaneau yachts were lifted out in Horta due to spade rudder failure. Fortunately nobody lost their life in the incidents and compared to the many yachts crossing the Atlantic the proportion of failure is small.