But it's true. And it's no wonder since the w row of kana has always been the tricky one. First we have
wa は, wait, no, that's the particle, I mean, wa わ. Then we have wo を, which is pronounced like o お when it's used as a particle, and it's practically always used as a particle. Then we have wu... wait, that one doesn't even exist! There's only u う and that's not in w row.
Then we have the wi ゐ and wu ゑ in hiragana and their wi ヰ and wu ヱ katakana counterparts, which look like kanji a little too much. These kana are archaic, obsolete. They used to be used in words (and names of people) but their usage has been discouraged since a Japanese language reform after World War 2.
That's why they are so rare. Because they are not supposed to be used at all.
On top of that, remember how I said wo を is almost always pronounced o お? Well, we ゑ and wi ゐ are officially pronounced as e え and i い. Words that used to have we ゑ had it replaced with e え and words that used to have wi ゐ had it replaced with i い. Examples:
- iru 居る (iru いる, formerly wiru ゐる)
To be (somewhere)
- koe 声 (koe こえ, formerly kowe こゑ)
- e 絵 (e え, formerly we ゑ)
Sometimes you may still find those archaic kana (like in the name of the mangaka 漫画家 of Nichijou 日常, Arawi Keiichi あらゐけいいち) but their usage in modern Japanese is pretty much gone.
Now you know the secret behind the w row and its rare, archaic kana.