winning 10/10 means nothing if you beat people who are all bronze 5. Especially if those people are the really bad bronze 5 or fresh bronze coming from unranked play. However, i concede that winning 10/10 shouldn't give you bronze 5, it should at least give you about bronze 3. If anything winning those games should place you against better bronze but then seemingly you should and would do worse in cs, kda, and, obviously as a result, the winning percent . Duo games might affect that guys placement as well, although i dont know the specifics.
The ranked system does work which is the problem. It just takes too much time to get to your "true" rank. However, this is expected because if a small number of games could find your true mmr it would probably just be too small a sample size. A small sample size used to determine your rank is bad because it introduces a large amount of error and random chance. Thus riot places you lower (rather than higher) because they think if you dont belong you can get out anyway and encourages actually playing ranked because you aren't stagnating or plateauing at your "true" rank right out of the gate. (again, if they accidently somehow guessed your true rank from the beginning then it was probably by extreme chance because after 10 games there just isnt enough data).
I got placed s5 last season, rose to g2 (only skipped g3 oddly enough), played on my smurf and got placed in s1 this time. My first game in s1 however, was against 2 p5s 2 g2s and a g3. Thats annoying, but acceptable since i have a true mmr around g2 like my main. This is only annoying in getting out of the rank s1, but isnt affecting my mmr much.