Berserker Leah is basically a weaker version of Berserker Freya, which class isn't even really suitable for the latter but instead more for Vincent. The aura is kinda weird for Berserker, you don't really want your Berserker to get hit because they usually get instagibbed since they are the either one shot somemore or get one shotted class.
Warlord Leah's Aura is too passive for what she offers in her active skill, which is to push a whole row back, but the issue is that she's melee, if she pushes them back, she can't really hit them next turn either, good to push away Paladins off the middle point but that's about the only niche purpose for this class.
Blowing 5000 blood tokens on her? Please just save up your extra 1000 tokens for a Valiant ticket or just trade for Siena/Kira or heck, get your wings ticket. She's basically a faith reroller and doesn't offer much for her price.
For the event, Slayer Sylvia looks like she could offer quite a lot based on her stats boost for the upcoming ladder with her Command Aura although the skill is just pure damage line, Lancer Sylvia is meh in terms of aura and skill.
Faye is something I would consider the most worthy to actually spend for out of the three new heroes.
Holy Defender taunts the entire team, removes active buff, and shield the entire party for 50% HP (about 6000 when properly pimped) although she has a pretty weak aura, I would say Holy Defender is the way to go overall.
As for Paladin, she has a really niche PvP purpose since she doesn't taunt at all but she beefs up the party by giving them all reflect and 30% HP shield, and the aura is pretty cool since it has a 75% chance to proc which is very high.