Valeera is always going to be niche due to her reliance on her openers, and there’s nothing wrong with that. If they were to buff her, they’d need to nerf the openers and buff her regular abilities, which for all intents and purposes is not what she was designed for.
Nerfing her openers would make her less viable than any other melee assassin and if they buffed her too much, she’d could essentially be an unfun gamebreaking hero, so I’m sorry to say this about what I assume is your favorite hero, but Valeera is niche for a reason and needs to stay that way.
As for Nova, she’s been bouncing back and forth in and out viability for awhile, like Anub’arak, she’s not an evergreen hero who’s going to be good in any situation. She’s got her niche too, but when she works, she works incredibly well, especially with the right player behind her and a good comp.
The problem with Nova, as with other stealth heroes, is how easily weak players can abuse the stealth to get rewarded for poor positioning and/teamplay. Maybe they’ll wait out until a teamfight is nearly over, risking the lives of his/her teammates just to secure a kill, instead of actively participating in it for fear of being focused. Or maybe they’ll sneak into the back line of the enemy team, gank the healer, and walk away in stealth Scott free from a team that just happens to have no skill shots or reveals to counter her.
The stealth changes from last year, from how I see it, was to reduce their reliance on stealth and put more power into their actual kits so that they’d be more willing to participate in teamplay, rather than play selfishly just for the opportunity of an easy pick that could have been achieved more easily if the stealth player was actively participating in the teamfight in the first place. For a time, the changes were relatively successful, since both Nova and Zeratul had both vastly improved from where they were before. That’s not to say that they were non-situational, but they were a huge step up from where they were before.
You just gotta accept it, no matter how many times they buff stealth heroes, they’re ALWAYS going to be situational just due to having to balance them around having the stealth mechanic.