This is true. I play alone and managed to rank from platinum up to diamond 4. Since then it's been slow so I'm guessing that's pretty close to my proper rank. Getting an Onyx playing alone will be extremely tough.
I'm the exact same. Started plat 1in both slayer and team arena. Solo queued up to diamond 2 and diamond 4 but the games are getting pretty tough.
Diamond 4 is about right for me... Especially since I dinged diamond 5 then dropped back down a couple games later