I guess this is one of the frustrations of playing a game like Chi. What is rare to one is common to another. For example, I worked most of my adult life in the medical profession where such words as 'medial', 'forte', 'nocte', 'ictal', and so on, are commonly used (certainly here in Australia that is the case). Now, I should imagine millions of people in the English-speaking world are employed in medicine and would be perfectly at home with some of these words in everyday usage. And, of course, this would play out in whatever field we work or play in. What's common to me is rare to you.
So, IF fairness (and I'm not sure it is, although I think it should be) is the criteria for climbing the Chi-scale of achievement then the game becomes unfair. But, do remember, it is JUST a game.
Similar games are offered in many daily newspapers and the way they gauge achievement is generally by a 'fair/good/better/best/genius' scale, much like the bottom of the Chi page. They don't adjudicate on what is common or rare. For me this is far less frustrating (and fairer). I have a better than average grasp of the common English lexicon, so I'm always in with a chance. I have a lousy memory to recall some weird words commonly played by others so tend not to use them.
So, this is why my preference is not to aim for a rosette or trophy and instead aim to get a 'best' score. This way I am satisfied, not commonly frustrated, and don't spend inordinate amounts of time trying to chase down elusive words, some of which are anything but common to me.
And I still play the game (usually, all three of them) daily. Actually, I think I'm addicted. Got to be a better addiction than drugs, gambling or pornography. Thank you, Alan, for a delightful daily distraction.