I mean, to some extent I don't comment much because, frankly, the boards aren't the best place for legitimate design discussions right now. A lot of our community is, understandably, highly outspoken about things that they think are problems, and are often quick to propose solutions that may not have been fully analyzed. I totally get that, but it makes me personally (as a very analytical person, sometimes to a fault) get sometimes frustrated with balance discussions here, as there tends to be a lot of emotional responses and not as much thoughtful analysis as I seek in such conversations.
That said, I think the balance team is sometimes a little too cautious, but is normally very good. A lot of the problems we see cropping up are typically more due to the evolving meta than any specific champion outlier (exceptions exist), and that's not something the balance team alone can really address. Some of it are the result of large ecosystems of the game (itemization, for example) that is being slowly improved as time, designer availability, and resources all become available, and some of it is improved by new champion designs that better fit with League's current design philosophies.
I think expecting the balance team to bat 100% is unrealistic on a game the size and scale of League, as every single change has dramatic ripple effects. Their track record is largely pretty good though, even if it takes a little longer than some people like, but that's a reality of design -- it's easy to apply a fast solution to a problem, but it's FAR more beneficial to wait a bit and apply the RIGHT solution to a problem. Even then, however, there will always be people that think any given change is the wrong change, and that's also just a reality of design.
Oh, and the reason for more nerfs than buffs for problematic champions is that champions who are very weak are often that way due to kit issues (such as kits that get too easily powerful when tuned to have solid damage, or kits that are just struggling to function), and buffing them can have serious consequences. Alternatively, you get into an arms race where you have to buff 50 champions up to the level of 4 outliers. It's FAR easier to prune the top and help out those struggling champions you can help out in a healthy manner than risk upsetting the apple cart to mass-buff all under-performing champions.