I work in software developement / IT consulting and 90% of the time its the consumers fault. Be it some bloat or maleware on their machine, be it firewalls not being configured correctly and so on.
Now obviously I don't work with Riot and don't know the details of how the client and/or the game runs AT ALL. I can only tell you that the fact that some people have more issues than others CLEARLY hints to the source of the problem being the Local machine and NOT the client itself.
I KNOW the client has some issues and encounter SOME of those myself at times (Shop not loading or skip waiting for stats) but I encounter them very rarely.
And considering what type of stuff people have running on their Machines without even knowing, its likely many issues lie on the consumer side.
And yes Riot is still the ones who probably SHOULD troubleshoot if possible, yet they physically can't fix all bugs. The environment is changing too quickly, which is btw the reason MOST bugs in MOST Software happen. Its something changing. Be it a Windows Patch, a Driver Update or a Firmware Update on your router. There are so many factors to consider in these things that just "fixing" them is really not as easy as it sounds NOR a realistic option.