From my perspective, from what it sounds I'm around your rank @Kathryn_James, or there abouts.... most people our rank need security. They need good heals, they need to have solid defence. Good dive comps decimate because of defences that don't adapt BUT... standard defences (Rein + another Tank, Mercy and Lucio in support, a Solider for the Pharah) do very well against bad players trying the "best" meta comp.
Ultimately, it feels to me that at Silver/Gold rank right now there are only two thing that your team needs to do in order to win.
1) Work as a team, wait to group up, don't go off on a rambo unless the point of your character is to jump in and jump out (and you actually do that)
2) Ensure that you have someone covering for the almost inevitable tracer backline harassment. Even this is optional based on there being a tracer!
I think what Sedo is saying is that there are some characters that are just better than others, but also that it doesn't matter if they're better if YOU aren't good enough (or your team doesn't do what your character would need them to do) to make it effective.
Honestly, if I see a team picking Rein, Winston, Mercy, Lucio, Solider and (insert any other DPS, or Mei, here) then I feel that is a team that will be hard to beat at Silver/Gold, unless they're dumb enough not to stick together.
Edit: Always exceptions of course, there are still people out there in silver/gold that know what they're doing, I've seen some amazing Sombras, Anas, etc.... but consistently more people seem to know how to play the first 5 of the list up there so it just seems like meta aside there is more likelihood of those characters being tougher to beat.