I also think the manner in which 6's and 8's both can tend to get controlling could be a potentially terrible match. I do better with 4's, 5's or 9's.
The *worst* match for me, though, is type 7. I know a 7 and he is my life-long nemesis to the point where he's the inspiration for the antagonist in my novel. I love 7s as friends but in a relationship it's like stuffing two male bulls in a tiny cage. What could possibly piss off an unhealthy sadist more than an unhealthy narcissist whose skin you can't get under, no matter what you do? Someone who thinks he's better than everyone and everything? I don't think I could deal with dating a 7 even if both of us were tremendously healthy. I just can't take the ego & arrogance. And I know exactly what all their "happy" little act is about and that it comes from just thinking they're above everything, and it just rubs me the wrong way in a relationship, probably because my father is a 7. Innocence, modesty and sweetness turn me on and melt my heart.
That being said my best friend is a 7, and in my later years I get along with my father tremendously; and if I ever was inspired by anybody, it's him. 7s can be *great* and I can love them as people - I just can't date them.
Though the control issues turn me off to relationships with 6's, I'd take a 6 over a 7 in a relationship any day of the week!