These were the heirs of Bittersteel, and discipline was mother's milk to them. -ADWD, The Griffin Reborn

One of the things that I think is constantly forgotten during the invasion of the Golden Company/Young Griff is just how big and strong of a fighting force they are, especially compared to other free companies.

The Golden Company: Size, Strength & Discipline

The Golden Company was reputedly the finest of the free companies, founded a century ago by Bittersteel, a bastard son of Aegon the Unworthy. When another of Aegon's Great Bastards tried to seize the Iron Throne from his trueborn half-brother, Bittersteel joined the revolt. Daemon Blackfyre had perished on the Redgrass Field, however, and his rebellion with him. Those followers of the Black Dragon who survived the battle yet refused to bend the knee fled across the narrow sea, among them Daemon's younger sons, Bittersteel, and hundreds of landless lords and knights who soon found themselves forced to sell their swords to eat. Some joined the Ragged Standard, some the Second Sons or Maiden's Men. Bittersteel saw the strength of House Blackfyre scattering to the four winds, so he formed the Golden Company to bind the exiles together.

From that day to this, the men of the Golden Company had lived and died in the Disputed Lands, fighting for Myr or Lys or Tyrosh in their pointless little wars, and dreaming of the land their fathers had lost. They were exiles and sons of exiles, dispossessed and unforgiven … yet formidable fighters still. -ADWD, Tyrion II

Size

The Stormcrows have 500 horse. The Second Sons have ~500 men. The Windblown ~2,000. The Company of the Cat has ~3,000. The Long Lances have 800. The Brave Companions ~100. ... The Golden Company has 10,000 men:

The Golden Company was ten thousand strong, seasoned and disciplined. None of them seamen, though. -ADWD, Tyrion VII

and:

Brown Ben Plumm bulled over him. "Your Grace, the Yunkish got three free companies against our two, and there's talk the Yunkishmen sent to Volantis to fetch back the Golden Company. Those bastards field ten thousand. -ADWD, Daenerys V

and:

The Golden Company is the best and largest of the free companies, but ten thousand mercenaries cannot hope to win the Seven Kingdoms. -TWOW, Arianne I

Makeup

Of that 10,000 we know of:

• 500 knights and 500 squires

We have ten thousand men in the company, as I am sure Lord Connington remembers from his years of service with us. Five hundred knights, each with three horses. Five hundred squires, with one mount apiece. And elephants, we must not forget the elephants. -ADWD, The Lost Lord

• 24 elephants (unknown # of men to use them)

The horse lines were to the north, and beyond them, two dozen elephants grazed beside the water, pulling up reeds with their trunks. Griff glanced at the great grey beasts with approval. There is not a warhorse in all of Westeros that will stand against them. -ADWD, The Lost Lord

• 1,000 archers (~333 crossbow men, ~333 Essos bowmen, ~283 Westeros longbowmen, ~50 Goldenheart bowmen)

Black Balaq commanded one thousand bows. In his youth, Jon Connington had shared the disdain most knights had for bowmen, but he had grown wiser in exile. In its own way, the arrow was as deadly as the sword, so for the long voyage he had insisted that Homeless Harry Strickland break Balaq's command into ten companies of one hundred men and place each company upon a different ship.

and:

A third of Balaq's men used crossbows, another third the double-curved horn-and-sinew bows of the east. Better than these were the big yew longbows borne by the archers of Westerosi blood, and best of all were the great bows of goldenheart treasured by Black Balaq himself and his fifty Summer Islanders. Only a dragonbone bow could outrange one made of goldenheart. Whatever bow they carried, all of Balaq's men were sharp-eyed, seasoned veterans who had proved their worth in a hundred battles, raids, and skirmishes.

• ~8,000 unknown

We don't know the makeup of the leftover ~8,000 men in the company. That said I think that the fact that they are able to maintain the discipline, etc. below shows they aren't just scrubs, but probably a combined mix of light cavalry, spearmen, swordsmen, etc.

Discipline

These are seasoned warriors that have known nothing but battle their entire lives:

They found the Golden Company beside the river as the sun was lowering in the west. It was a camp that even Arthur Dayne might have approved of—compact, orderly, defensible. A deep ditch had been dug around it, with sharpened stakes inside. The tents stood in rows, with broad avenues between them. The latrines had been placed beside the river, so the current would wash away the wastes. -ADWD, The Lost Lord

and:

Tall battle standards of cloth-of-gold flapped atop lofty poles along the perimeters of the camp. Beneath them**, armed and armored sentries walked their rounds with spears and crossbows,** watching every approach. Griff had feared that the company might have grown lax under Harry Strickland, who had always seemed more concerned with making friends than enforcing discipline; but it would seem his worries had been misplaced. -ADWD, The Lost Lord

and:

Fortunately his own ship had been one of the first to reach their destination. Then it had only been a matter of establishing a campsite, assembling his men as they came ashore and moving quickly, before the local lordlings had any inkling of their peril. And there the Golden Company had proved its mettle. The chaos that would inevitably have delayed such a march with a hastily assembled host of household knights and local levies had been nowhere in evidence. These were the heirs of Bittersteel, and discipline was mother's milk to them. -ADWD, The Griffin Reborn

Experience Against Unsullied

According to the semi-canon app, the Golden Company sacked Qohor when it refused to honor its contract. Due to Qohor being defended by Unsullied:

Strong stone walls protect Qohor, but the people of the city are not of a martial bent. The Qohorik are merchants, not fighters. Apart from a small city watch, the defense of the city is entrusted to slaves—the eunuch infantry known as the Unsullied, bred and trained in the ancient Ghiscari city Astapor upon the shores of Slaver's Bay.

It seems they have experience fighting/defeating the Unsullied.

As Arianne states "10,000 mercenaries" cannot take the Seven kingdoms, just like 10,000 Dothraki screamers couldn't do it. But like the Dothraki the men of the Golden Company aren't raised levies or tourney knights, but seasoned veterans who have known nothing their entire lives but war. Obviously not all of them have reunited yet in Westeros, but I think most will in time.

TLDR: The Golden Company won't take Westeros alone, but I think the size, strength, discipline of the Golden Company is often underestimated as they are bigger than all the other sellsword companies combined.