Two words: Hits hard.
That's a simplified explanation of it, but Cruel Blade rocks a buff called Heartless from parry, and optionally parry break, that ups its damage by 10% per stack, up to 6 stacks. Each attack eats a stack, so it isn't going to be up at full power for every hit, but the frequency and ease of generating stacks for an invested set means it's going to be doing some major damage before critical hits even factor in.
The other thing the set does is spam stuns liberally, but only if invested, before which point it is a lot less effective. You'll see this in action if you use a fully 8-pointed set in Hua, but getting at least 130 frags to make the set stun in the outdoor world is more or less mandatory if you plan on using it. The downside of this is the ability to stun takes points away from your ability to generate Heartless stacks as you level it in the early game. This is another reason you don't tend to see it in the outdoor world as much as Snow.
Compounding these two factors, the set attacks at a fast pace and once an opponent in the air it can keep them there for a respectably long time. Not as long as a Wudang sword, but enough to deal some serious damage.
Cruel Blade Skills
Severing Ties with Rage is the auto. It benefits from buff stacks and can knock enemies into the air if yellow armor is down. It can charge an opponent from fairly far away but does this less consistently than some sets.
Crossing One's Heart, your feint, can charge opponents but it's slower than Wudang's feint charge, it makes you do this weird somersault and a sort of slow cleave. It can be hard to land this thing, but it will paralyze the opponent for a second if you do, 3 seconds with any point investments, and up to 3 stacks of Heartless Buff at rank 4.
Your parry, Merciless World is the main generator of buff stacks, but will also quickly eat them with the counterparry. Maintaining buffs with this thing requires quick timing of fast parries before an opponent feints. It only generates one by default but can generate up to 5 stacks per each parried hit at max rank. The max rank of the parry will also charge at opponents.
End of Love is a two-hit knockback with a 3-meter range if uninvested. With 4 points, it's an 8-meter range with a 3 second stun and a knockback. Essentially, it's an even slightly better version of tang dagger stun, allowing it to stall opponent momentum.
Heart Breaking Blade is a long-duration frontal cone that can also charge opponents. It will knock opponents into the air if they don't have yellow armor, meaning it's used after a parry break or successful counterattack.
Your rage, Bloodlust, will gain red armor if used out of range of your opponent and charge to them automatically. If already in range it will be a yellow armor move. The last hit can throw them into the air. This is somewhat hard to time and typically only happens if used immediately after a parry break. If they are already in the air, they will be kept in the air for the duration of this move.
With the mechanics and the general jist of the set explained, there's really only one last thing to touch on and it's how you invest the points. As mentioned, the stun cleave is more or less mandatory to make the set function as anything more than a hard hitting counterparry. The parry is critical to generate buff stacks, but you'll notice very little meaningful difference between a rank 3 and rank 4 parry. Rank 4 can charge opponents, which can be a mixed bag as the aiming of that charge can occasionally be less than stellar. Not taking rank 4 lets you extend your parry break from 1 second of paralysis to 3 seconds, but the difficulty of landing the parry break can make this difference somewhat minimal or hard to appreciate.
Complimentary Mental Skills: Royal Guard Mental, Tangmen Mental, Steady Accumulation, The Steadfast, Explosive Thunder. Anything to make those nasty buffed hits hit/crit even harder.
Tl;DR: Learn Cruel Blade if you like to hit very hard with short reliable combos and harass opponents with stuns, but beware its investment costs and what your time levelling it could do elsewhere.