My biggest problem with Bitcoin is, I don't know where the easiest and the safest place to keep it.
I want to use the Bitcoin core, but my PC couldn't handle it. So I use an online wallet, but I don't know if it's safe enough.

I have no doubt this is the biggest issue with bitcoin.
Except for people looking at it from the outside who never actually owned bitcoin. They may see the volatile prices or lack of backing etc. But the actual real main concern is the security issue.
it's not the biggest issue at this moment, but with the increasing blockchain size it will be much harder for people to make space for it on their hdd in the future.