I’ve noticed some weird things with $user_nicename particularly on some single WP installs.  For some, the user_nicename had spaces which broke member profiles.
So I’ve had to stick to the compatibility define in some cases.
Via the commented code in WP:
'user_nicename' - A string that contains a nicer looking name for the user.  The default is the user's username.
		
	 
	
	
	
 
		
			
	
	
		
		Can somebody help me with this nicenames business, too?  I’m working on the username/displayname oddity I’ve already been chatting with  @r-a-y about (re: the username only plugin) and have presently disabled his plugin and added this to my bp-custom.php:
Example 1
Well that’s lovely at various points across my site, but it doesn’t add the title info to links in, for example, the members directory.  (Perhaps I should’ve more deeply investigated using your plugin as a base point, r-a-y!)  So now I’m messing with the file members-loop.php, but the file bp-core-templatetags doesn’t have anything to handle whether BP_ENABLE_USERNAME_COMPATIBILITY_MODE is set, and consequently I’m not sure whether to request bp_member_user_nicename()  or bp_member_user_login().  (as in:  Example 2).
Obviously, as I’m currently set up, I know that I actually need to do the nicename thing, but on a more abstract level, what should I be doing?  I see there’s a filter in function bp_get_member_user_login() I might be able to monkey around with, but as it’s only passed $members_template->member->user_login, I’m not sure that’d work (or, ignorant question, is a global variable really that global?).
I’m a bit over my head, in case that’s not already really obvious.
Would the best solution involve disabling these nicenames, whatever exactly they are?  ( @r-a-y, I see the comment you pulled out of the WP code, but it doesn’t *mean* a lot to me.  In my case, I’m not actually even sure how the two got set up, since I didn’t do the initial install of WP on my site.  I just know the administrator comes over as [at]admin but with a login of cltgeeks, and I’ve sussed out that admin is evidently the nicename.)