Wow, big discussion now here
Speed: Like Ch3vr0n already said, rated speed could be better than a lower speed. It depends on the writer and the media. BDs are generally specified with a low speed, so you have a real bad drive or real bad media if you only get an successfull burn at lower speed. Overall best drives are Pioneer, thats my experience with five brands and 10 (3 LG, 2 Panasonic, 3 Pioneer, 1 LiteOn, 1 Samsung) models.
I test some media with burning speed 8x and 10x and with good media there´s a small difference, no Standalone-BD-Player would make problems with these faster burned media.
DVD-media in these days are mostly rated 16x and indeed, only very good media (which is hard to find now, many manufacturers stopped production, others are dropped the quality to reduce costs) will like it. If you turn down the speed to 8x or 12x you will get an good burn. Actual DVD-writers will offer minimal speed of 4x (Samsung), 6x (Pioneer, LiteOn) or 8x (LG) with 16x media. 4x and 6x will not have better results than 8x.
I never burned BD-DL-media, so I can´t say something about the Ridata-problems.
But Ritek-media like Ridata, also some models of Platinum, Maxell, Primeon, are one of the worst media you can get. It was good directly after burning, but I post here are screenshot how worse it is after some years.
Most ppl would say it is a successfull burn if the writer don´t show an error-message, other ppl would say if the BD-Player plays it. I check the write-quality after the burn, maybe also a transferrate-check at max. read speed; I make after some years a re-test of it. This is a much better indicator of the medias actual condition.