Slave Rebellions Always Fail
I can not emphasize enough how often they fail. In the thousands of years of human history where the majority of civilizations employed large scale slave populations, a slave rebellion has literally never won a total military victory and secured themselves as a sovereign nation all on thier own. The very few successful slave insurrections such as the Haitian Rebellion were only won with the heavy support of foreign military and logistical aid.
A lot of this boils down to how big of a difference proper weapons, armor, training, and tactics makes. Boudica's Revolt is a good example of this. While Boudica's army was estimated to be at least 8 times as large as the Roman army, her rebellion was still a total failure. The big issue was that most of her army was just peasants sent to the battle field with whatever improvised weapons they could muster. The issue with sending poorly trained and equipped troops into battle is that most individual soldiers care if the battle will be won or lost a lot less than they care if they will survive said battle. When the superior Roman weapons and training led to the decimation of Boudica's first few lines of troops, the guys next in line saw how badly they were losing and tried to run. As they broke, the guys behind them saw things were not going well so most of them tried to run too. Those who may have been brave enough to stand and fight were no longer in a proper battle formation, making them easy to isolate and kill. This sort of panic is called a rout, and is the real winner of ancient and medieval battles way more so than just fighting until all the enemies are dead. The routed army suffered 80,000 casualties as Roman soldiers and cavalry chased down the panicked rebels. In all, Rome only lost 400 men.
The difference in quality of arms and training between an army and slave tools only grew larger in the Medieval period. For a slave to even hope to beat an army fielding armored knights and men-at-arms, they needed weapons and tactical knowledge that they generally did not have. The longest improvised pole-arm a slave could muster would be farming implements no more than 4-6 feet in length, and they would have known nothing about pike blocks or phalanx tactics to even know that they needed more. They would very rarely have any access to shields, armor, or bows of any sort, and a knights lance was typically 10-15ft long such that knights could just charge down rebel slaves with impunity, shoot them down with bows, or dismount and decimate them using shield wall tactics.
All this said, there was one weapon easy enough for slaves to secretly make a lot of that could threaten a mediaeval army: the sling. A sling can be fashioned easily from found scraps in what little free time a slave may have, it can be hidden very easily, and trained with quietly. On top of that, slings are excellent weapons against armored enemies. The Romans for example actually preferred slings over bows as weapons of war because the impact of a sling could kill a person from internal injuries, even when the actual stone failed to penetrate the armor. While a bunch of poorly trained slaves with slings may still not be able to beat a professional army of archers, pikemen, and cavalry, they will still be able to inflict enough heavy casualties to make their masters understand that the sling is the most dangerous of all slave weapons.
Why removing fingers does not work
In the Medieval period, the French would allegedly cut off or cripple the ring and middle fingers of captured English archers to prevent them from ever using a bow again, but crippling a slinger is much harder. When using a string, you typically pinch the release string between the index finger and thumb, and the stay string is either looped on the middle finger or pinched by the lower 3 fingers... however, typical is not the same as the being the only way to do it. Longbows had very high draw weights that made them pretty much unusable with the weaker lower fingers, but slings don't put nearly so much stress on the hand for equivalent stopping power. If you remove the index and middle finger, they can just loop the pinky and pinch the thumb to ring finger... in fact, with a bit of ingenuity, a sling can be used even with any 4 fingers missing. Cutting the fingers off of a rebellious slave actually reduces his options for what weapons he can wield directing him towards the dangerous sling.
For a slaver, it is in many ways better to let a slave have all 5 fingers, and to cut him down when he tries to fight you with a pitchfork, than it is to cut off fingers forcing him to consider his other options.
This is before you even consider how much missing fingers devalues your slaves. Also, dismembering your slaves as a matter of policy is a good way to trigger a rebellion, not prevent one. If a slave thinks he can have a tolerable life by doing what he is told, then he's a lot more likely to be compliant than if he knows he will be maimed either way. Using violence to control people only works when you leave some hope that they have the means to avoid that violence by submitting to your control.