tldr: class masteries are ONLY useful if you use that class regularly and they're only useful against SOME enemies. if you have leftover mastery points and you use a certain class often, these are worth it.
long version: they're SUPER niche. like, super super niche. when it applies, it's awesomely useful. when it doesn't, it's waste of mastery points.
mystic: Your Mystic Champions accrue x% (2-12) Power each time an enemy beneficial effect expires.
this is pretty useful since many champs use beneficial effects.
but this is absolutely useless against enemies that don't use beneficial effects: BP, unduped bolt, DPX, ant-man, captain america, elektra, electro, magik, magneto, ronan, storm, etc
cosmic: Imbue Cosmic Champions with perception rivalling Heimdall, reducing enemy Armor Up effectiveness by x% (2.5-40).
pretty useful against kang, col, thanos, war machine, IP, S/IM, etc.
but totally, absolutely, without a doubt useless against literally everybody else.
tech: Retrofit Tech Champions with a field that inhibits enemy Power Gain by x% (4-18).
this is one of two class masteries that's universally useful. but 18% isn't too noticeable or game-changing. not to mention, if you're using the/vision, you WANT your enemy to gain power so you can keep draining that bitch. if you slow it down, you're slowing down your damage. as a note, 18% is about the same as moon knight's sig lvl 8.
science: Give Science Champions a Serum of +x% (2-32) Energy Resistance.
pretty damn handy against ultron, the/vision, miss/cap marvel, storm, BB, ronan, thor, cyclops, dr. strange, witch, etc.
but useless against physical champs. but admittedly, many champs have at least one energy special. but enough champs are purely physical that this is considered niche.
mutant: Rapid mutagenesis closes Bleed wounds in your Mutant Champions x second(s) (0.5-2.5) faster than non-Mutants.
colossus doesn't even benefit since he doesn't bleed in the first place...
it's really REALLY useful against hawkeye, gamora, drax, elektra, war machine, fix-it, wolvie, BP, guillotine, rocket, deadpool, dpx, ultron, etc.
but useless against... literally.. everyone else.. literally.
skill: With extensive field training, Skill Champions ignore an additional x% (4-64) enemy Armor with Critical Hits.
this is the second of two masteries that is universally useful. some champs don't use a lot of armor, sure. but this means some kind of permanent damage increase.
against colossus, war machine, IP, S/IM, kang, thanos, etc this is devastating.
especially with black widow, elektra, BP who use crits like they breathe air, this mastery makes them fish in water.
each one has their use. but this "use" only applies to certain class types and only against certain enemies.
if your team is mostly one class and you have mastery points leftover, i think these are great. also, if you already got cruelty/precision and you're saving for advanced masteries, class masteries can be a good transitioning mastery.
but bottom line, there are few cases where i would make these my first priority. restrictions to certain class and certain enemies are too niche..