A crossbow has several advantages, but it's main advantage is NOT power or armor piercing ability. That is likely a myth perpetuated by armchair historians with poor grasp of physics.

The main advantage of a crossbow is that it's easy to use. Richard Lionheart, the English king, was famously killed by a boy wielding a crossbow. You need no more than 2 weeks to be proficient with a crossbow. To do that with an English longbow you'd need 7 years.

There are various methods of drawing a crossbow that don't require great strength. Here's a (not exhaustive! Missing is for example latchet crossbow.) list of 9 ways to draw a crossbow:

Tod's Workshop

9 MEDIEVAL CROSSBOW DEVICES - How do they work?

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https://www.youtube.com/watch?v=2IdfmaC_t-Q

In the video, Tod Cutler shows 9 mechanisms used to span a crossbow, some of them require a specially modified crossbow and some don't. Crucially, he shows the numbers for mechanical advantage. Keep in mind the input number will vary. Cranequin is an amazing mechanism, but you only use a single hand. A windlass doesn't have such an impressive ratio, but you can use both hands. A belt may let you use your back muscles as well.

Videos of Joe Gibbs, a 170lb man who can draw a 200lb longbow, show how much effort it is. In one of the videos - I can't locate which one it is - he explains it's no coincidence medieval illustrations show people bending forward when drawing a bow. He draws this way and it lets him put a little more extra into the arrow.

Crossbow can be kept loaded for a long time, and you can line up your shot. This is especially important in sieges. Also, they have a controlled release. My observation is that missile weapons evolved to have more and more controlled release over the ages. Slings and atlatls required lots of skill to use, bow is a bit more controlled, crossbow even more controlled (less things to mess up). Slings were even more powerful than bows but even more training was required.

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Now to the power issue. The myth is that they hit extra hard. A light crossbow (loaded by hands, or using a goat's foot lever) actually has power comparable to a short bow. Something like 60-80lbs. Witness a nominally 350lbs failing to penetrate a steel helmet:

Can a 350lb Medieval Crossbow Penetrate Steel Helmets? (16 / 14ga)

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https://www.youtube.com/watch?v=UqGZl5MVFPg

Why? The answer is short power stroke. Just because a bow or crossbow can be drawn with a certain force doesn't mean they impart that force to the arrow! Power stroke is the distance between the end of the bowstring, when fully loaded, and between the front of the bow. Crossbows are inefficient when it comes to transferring energy, especially the ones made using medieval technology and metallurgy. Steel bows are nowhere near as flexible as good wood or modern alloys. This was proven time and again using a chronometer (projectile speed meter).

Heavy crossbows, meaning those loaded with a windlass, hit about as hard as a war longbow, which is 100-120lbs. My numbers are from Tod's Workshop youtube channel.

Both a longbow and a heavy crossbow are strong enough to pierce mail armor - piercing is actually its weakness. Mail is good against slashing blades and blunt objects. Even a light crossbow punctures mail. But they fail spectacularly at puncturing plate:

https://www.youtube.com/watch?v=Ej3qjUzUzQg

https://www.youtube.com/watch?v=XMT6hjwY8NQ

And a longbow:

https://www.youtube.com/watch?v=DBxdTkddHaE

And here is the most powerful traditional crossbow I've ever found on youtube

Shooting a 1257 lb Great Horn Composite Crossbow - version 2.0

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https://www.youtube.com/watch?v=AA5M0QKXtWU

It is 14/15 century, so pretty late medieval tech (Swiss). Crossbows don't get bigger than that. It's practically a very mobile siege weapon.

Another downside of short power stroke is that the crossbow bolt (arrow) accelerates much more rapidly than a bow arrow. So it must be sturdier to withstand the forces, which is why bolts were so thick. A thick projectile has more air resistance and worse armor penetration.

So yes, crossbow is the RPG weapon, but in the sense of "Role Playing Games" and not "Rocked Propelled Grenade".

And here is how much of expertise you need to make crossbows the medieval way:

A Castle Fit For Fighting | Secrets of the Castle (2/5) | Absolute History

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https://youtu.be/u6v-3Ai88oM?t=1218

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I hope I've proved that power was not the advantage of crossbows. The weapon had many advantages, and the basic shape of the weapon and its trigger is used to this day in rifles because it's so good.

If power is what you're after, check out staff slings (Fustibalus). This thing is SCARY, it cracks coconuts at a distance.

Slingshot VS. Shepherds Staff Sling

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https://www.youtube.com/watch?v=_bFQYNfJqqY

This is a very solid video with measurements.