I find only 6 twists on the fluoro or mono is more than enough and about 12 twists with the braid is good when doing the double uni. It tends to be the mono that fouls on the guide so try to make the mono side of the uni knot small by giving it less twists. This is for 30 to 40 pound mono. Any thicker and you will def need bigger guides.
Cheers Andrew
Sorry. I forgot to say the double uni is very safe. A fishing guide in cape York used it and he helped me catch some good barra, queenies and trevally with no failures. The trick I suppose is the rod tip guide size to the size of the leader.
The key is to have the right rod size for the line and the guides on the rod will suit.