There have been tests by PC per into framepacing for both Crossfire and SLI, and SLI more often than not is a higher framerate version of the single GPU.
And as someone who has used SLI for 10 years, your claims are rather outrageous. The most recent problems of SLI not working in a number of high profile cases does not reflect its entire catalogue of games. Hence why there are huge threads on Guru3d or, 3d center, or computer base dedicated to SLI users cataloging older games and newer games in SLI configs.
Of the last 5 games I have played recently - Wolfenstein TNO, Darksiders, Alien Isolation, MGS GZ, and s uper old DX 8 game, Ravenshield - only 1 hasnt had working SLI (which is an engine problem, not NVs). The rest scaled perfectly with TriSLI.
Your opinion doesnt seem reinforced by any proof or even any recent anecdotal evidence.