your assumptions are correct. the Eno hub axle defines a circle around the attachment points and the consequences are correct as stated.

i don't have a problem with my two bikes that use it, but i don't run fenders or a rear brake. alternatives involve a dedicated chain tensioner or eccentric BB or compromises concerning gear ratios ("magic gear", see Sheldon B.).

BTW, the amount and frequency of chain tensioning necessary to effectively MAINTAIN an existing gear ratio is negligible. i find most if not all the fuss involving the ENO hub is when deciding upon the proper chainring/cog/chainlength issues, which, if you are like me, is not very often. once set up with the ratio desired, the wear on a chain or cog or chainring that demands hub re-positioning has little real-world effect on "attitude" changes or brake positioning (which is trivial).

and i can't say whether or not you are "missing something", but i can't find my 3/8 socket wrench. you don't, by any chance, have it, do you?