Most threshold’s require some degree of sacrifice and Specialization. To list some of the more powerful ones:
Alpha: 15 in all stats - no real downsides - can wear everything
Elf: +4 to all stats (+5 dex) - perfect night vision - sleeps once a week - can wear everything
Medical: Heals ultra fast - Immune to everything including pain - can develop schizophrenia and some wierd addictions - main drawback is that the best mutations are post thresh and it locks you out of other thresholds.
Squid: Requires dex build - boosts dex to absurd levels (20+ potentially) - gets like 11 attacks per turn - breathes under water - needs alot of water - needs warm climate - can’t wear anything.
Slime: Requires int build - boosts int to absurd levels (20+ potentially) very resistant to toxins and radiation - can spawn minions - can’t wear anything - can potentially get genetic chaos mutation.
Bear: Unkillable super tank- very strong - big boost to combat skill learn speed - can hibernate - has trouble fitting in cars - can wear very little
Spoiler: Huge quality of life improvements (food and water issues completely resolved) - Can insta eliminate the more powerful undead enemies (at the expense of giving basically no advantage against non zombie enemies and gimping the loot drops of the undead in question) - fire resistant - slow - can wear everything - can potentially require a long world spanning journey to acquire.
In my opinion these are the only mutations that are worth it to cross the threshold in. Alpha, Elf, medical, and spoiler are basically human+. They give good benefits without severe disadvantages. Squid and Slime have crippling disadvantages, and require you to adjust your playstyle around them, but they can be absolutely gamebreaking when paired with the appropriate martial arts and stats. Bear is the only “beast” type mutation who’s strengths compensate for it’s weaknesses.
The rest have very serious issues:
Lizard: gives inferior versions of other mutation lines
The Beast Variations: crappy combat bonuses in exchange for the inability to wear armor
Bird: Dies in one hit, makes late game content hard to experience
Insect: Literally can’t eat during the winter. Nuff said.
Plant: Can’t raid underground facilities without suffering ridiculous penalties and most likely starving to death.
Troglobyte: Disintegrates in sunlight (has great pre thresh mutations though)
Spider: Mediocre abilities that are all over the place
Chimera: Requires you to do nothing, but kill, eat, and drink nonstop.
Raptor: Hits hard but dies in one hit and wrecks armor slots.
Hope this helps.