I wonder if it worth my trouble buying from KEH?
That's something only you can decide. Personally, I bought from them few times and returned few items. I ran into issues on one transaction. Seems when things fall out of cracks in their system, it takes forever to resolve it. But eventually, I got things sorted out. Despite this, I plan to keep buying from KEH.
Keep in mind, KEH deals with used items. Despite its best intentions, hidden issues or intermittent issues may remain undiscovered. For that, it offers generous return/warranty policy for used gears. The most you can lose is the shipping. Also, there may be a staff or two who really don't care or pay attention. That happens too.
I had issues with other vendors and even with new items, so having issues aren't unique to any business. I'd rate my personal track record with KEH to be "good" service wise and product wise.
If you ask about any product or business on forums (of any kind) you will get all kinds of responses. According to many, Nikon, Canon, Toyota, Honda, Kenwood, Hargen Daz, Coke, Pepsi, etc, etc, etc, are the worse company/worst product ever produced/invented. Personally, I take any opinions I see with large dose of skepticism as I have no idea on qualifications/standards/motivations of individuals making comments - good or bad.