I’ve been trying the six-letter version to see if it’s more challenging than the five-letter one. Yes and no. I can cover all five vowels, the letter Y, and six common consenants with two guesses - ADIEUS and PORTLY. An average of 3 or 4 colored letters will show, but it’s still a challenge to put the yellow ones in the right order. If you have only one vowel, most likely it occurs multiple times (unless the solution is something like LENGTH).
The only ones I’ve missed (six guesses, like the five-letter version) out of 30 or 40 that I’ve tried, were words in which I was missing one letter with one or two guesses to go, and i guesses wrong. My ratio of 3s and 4s is about the same as with five letters.
The sixth space gives you a chance for an extra letter guess - eliminating more consenants.