I don’t think the ‘talent and ability’ picture supports adding health to Slayer and Elf careers, and your own examples are a good place to start, because mobility ults and talents aren’t the only variables that matters. The whole kit is.
Take Footknight. His ability is a charge, but that’s mostly stagger aimed at disabling enemies and reaching a teammate who needs rescuing. And across the board Kruber’s weapons are slower, higher-stagger, and lower-dodge than the elf’s. Same for the dwarf. So Footknight’s 150 HP isn’t “mobility ult plus high health”; it’s high stagger, a control-oriented ult, and a defensive weapon pool, with health/DR to match the trade-hits playstyle. He also has fewer talents aimed at dealing damage than Handmaiden, but I’ll grant you, they are about as similar as two careers can get, except Foot Knight is more stagger/DR, and Handmaiden is more damage/mobility.
Zealot’s not precisely a standard tank, he’s a risky tank. Either you keep your green HP and play a mediocre tank, or you go for THP and get both tankiness and damage, but you carry the risk of losing your THP if you can’t maintain it. His 150 isn’t free durability; it’s the resource the whole gamble runs on. This is similar in nature to Unchained which takes less damage and can deal a good deal of damage, but that comes with a great deal of risk since taking damage can overcharge her and kill her.
Slayer may only have 125 health, but look at what he already gets: extremely effective DR talents up to 40% on charged attacks or half damage taken from elites/monsters plus Leap which can stagger nearby enemies. His talents make him easier to keep alive, not harder. He already has the survival tools layered into his offensive capabilities. Getting a ton of attack speed on weapons capable of stagger and killing elites fast makes him able to easily maintain THP and control crowds if not just kill them outright.
Now the elf side, where I think your model is missing the biggest variable: the weapons. Every elf weapon has excellent dodge count and distance. That’s the throughline of her entire kit as I’ve said many times now. She trades stagger for speed and evasion, and her HP is balanced against that. On Handmaiden it’s compounded further by the dash, the extra stamina, and stamina regen. Adding health doesn’t reinforce a weakness; it stacks more survivability onto the most damage-avoidant kit in the game. Not only that, but her weapons have fairly high DPS.
You can see the same logic hold on the other ranged careers. Ranger Vet and Huntsman don’t get elf-tier dodge, but they get movement speed and/or DR talent options, they can carry a shield, and their weapons out-stagger the elf’s, so they’re ranged, with less mobility but with the crowd control to clear their own space. Pyromancer gets real mobility from her ‘crit to movement speed talent’, and her crit chance sits around 25–50% most of the time. Pyromancer also can get health from her ult and Sienna’s weapons offer moderate mobility and stagger.
On your “complex playstyle = more punishing mistakes” point, I’d argue that’s the intended play style of these careers. The fragility is what makes a wrong-direction dodge on Shade matter, but she’s also handed on-demand invisibility and some of the highest melee DPS in the game as the payoff. The elf’s high damage output is the reason it draws so many scoreboard circle chasers in. Shade and Waystalker are deliberate glass cannons (melee and ranged respectively). That “2–3 mistakes a game” of forgiveness isn’t for nothing, on kits this strong it meaningfully shifts the risk/reward the careers are built around.
On another note, when I first played Legend years ago, I was convinced most careers under 150hp (or that couldn’t generate massive THP fast) were just bad to play. I complained about it, and avoided squishy careers, that is, until I saw someone else surviving better as a Bounty Hunter than I did as Ironbreaker. That’s when I realized I was missing something. So I played Bounty Hunter a lot, and died a lot, but eventually I talked to other players, learned how to set up my properties and traits, got familiar with positioning and dodging, and I realized that it just takes time. There are 4 careers each on 5 characters, that’s 20 different ways to play the game and they each have different combos of talents, weapons, and properties. It’s complicated, and having mastered one doesn’t map well onto mastering another.
I think a lot of disagreements boil down to people having more or less experience with different characters. When something is genuinely weaker, on average, you’ll find that the people who put the most time and effort into the game will acknowledge it. Handmaiden is considered the most survivable career by many players I’ve talked to who have thousands of hours in the game, many more than me. This may seem like an appeal to authority, but it genuinely is how skill-based game communities sort these things out.