Legends is a brand for Disney equal to "not ours", within it there's various timeline, more details here : http://www.starwarsfanworks.com/timeline/download.html
. You'll see they wanted to merge it to a point it became unbearable and that Nathan P. Butler, but also Leland Chee, had given up to that and put two separate timelines to chose. Lucasfilm didn't bother to make it full and exclusively canon for their new Story Group because of the multiple novels and comics related to it they didn't want to add to their canon. But it's hard to deny this serie was a reboot of the Clone Wars period pre-2008 and they tried to fit everything with forceps to not displease the fans (that didn't worked). So instead of "Legends", you may read "former storyline" or "pre-2008 Legends timeline"
Contradiction or replacement are not indeed unique to TCW but the serie created more than 90% of them as Filoni liked to take content from the EU without respecting it. Unlike the other authors (including Lucas) who created their own things that sometimes contradicted with previous stuff. Sort of difference between malevolence and inefficiency, there's one I can stand, not the other. The worst is that he then defended himself by "all had been approved by Lucas". Other authors like Veicht and KJA used to call for Lucas to allow them more liberties but that was to create for in depht stories and more original content, not to twist other people's creations.
Little things from TCW would not bother me but that change for Ventress... That's not the same character in fact ! Not the same species, not the same physical appearance, not the same personality, not the same (back)story,...
For the EGtW, I've got a quick look, not seen anything regarding Ventress but the Battle of Boz Pity is mentioned and if going with TCW, this battle couldn't have happened the way it did in the Republic
comics serie for Ventress.
Well you do what you want, that's your mod after all. You'll even likely to get more public with more TCW content, with all these young who grew up watching it. The point of my question was just to know if there was ingame motivation for this description or if you just took inspiration from Wookieepedia (which replaced pre-2008 stuff by 2008-2014 stuff when the two contradicted).