This discussion comes up 2-3 times a year here. I always describe it as a problem of adult pelvis shape vs baby pelvis shape. They're round and we're boxy. Tapes need to (1) seal around the legs and (2) keep your diaper up on your hip. Using a single tape on a side to do both requires considering both needs at the same time. With adults, the legs and the beltline are pretty far apart so a single tape is going to have to compromise between the ideal positions for around the leg and around the waist. This may land well below your beltline, and if its too far down, your diaper won't stay up. If it's too high, you'll leak badly.
I think most people that have issues with cushies have issues with leaks because they're trying to keep their diaper from falling on the floor around their ankles.
So my take on it is that single tapes are just a cosmetic thing, get them if you specifically want your diaper to have single tapes. If you want actual performance, leak protection, and can wear while active without bruising your hips, go with doubles. While there are certain things about diapers I'm visually picky about, tapes ain't one of them, so I go for performance. I want to be able to go to bed, be comfortable, and wet my diaper all night long instead of getting up to go, and not worry about leaks. I just don't see that happening for me with a single tape diaper.