I think the goal should be to have multiple playable/viable archtypes for every faction.
So you cant have to many flexible leaders, because then all decks will play the same strong cards and the leader that benefites this playstyle the most will be picked the most.
At the same time you cant restrict the leaders to much, because then they autofill the cards and there is no room for creativity and experimentation.
The leader update sadly failed on both ends.
Now we have a bunch of restricting leaders aiming for the same mechanic for some factions and flex leaders for other factions.
First of, i think ST and MO are in the best spot leaderwise atm, their leaders are more diverse than other factions and encourage the usage of different archtypes and cards. for ST we have Harmony, Gord/Symbioses, Elf swarm, Dwarves, Shirru - while not perfectly balanced, they can all be played with some success. MO has Thrive, WH, Deathwish, maybe Swarm archtypes that get supported by different leaders.
SY, NG, NR and SK have 2-3 different archtypes that are playable, but most leaders aim for the same synergies.
SY has the hoard/engine package that gets supported with minor changes by most leaders (4 out of 7), gord/crime and swarm are the alternatives.
SK has the bloodthirst/warrior control list, that can pick between 5 leaders which are totally interchangable aside from strength, making most of them utterly pointless. Gedyneith/druids and selfwound are the only solid alternatives.
The same can be sad for NRs engine/boost archtype. Here we got 4 leaders competing for the same decklist, while stockpile is just plain worse by miles because it cant even protect those engines. zeal is playable. and thats about it.
and Lastly NG.. here we got a bounch of control lists, all almost identical. double/single ball doesnt really make a difference, the gameplan is the same. argueably 4-5 leaders support this archtype.. assimilate and hyperthin have some support.
now what did the update change about this, what variety do the new leaders bring to the table?? you are correct. absolutly nothing new.
In the contrary, it DID remove midrange control with vicious slash for NR and some meme miracle decks for SK. Aswell as the targeted abuse decks like kikimora and stuff (which i do support).
This leaves us at a much much worse state then before. only ST and MO are in a "healthy" spot leader wise, but Mo's balance is completely broken, almost removing them from the game..
So the TLDR: ST and MO have the best designed leaders, but the MO balance is broken.
SY, NG, NR and SK have a bounch of leaders that target the same archtype, resulting in decks that are basicly the same and leaders that dont see any play whatsoever because they are simply outclassed.