I believe Hand Cannoneers have 75% accuracy, Janissaries have 50%, and Elite Janissaries have 65%. Now that is at their max range (7 or 8), so I’m not sure if Janissaries are more accurate than Hand Cannoneers at a range of 7 or below. In general I’d prefer units to walk 1 tile and hit their target than to stay far and miss, that’s why Indian Hand Cannoneers have problems.
That’s a good point about Mangonels in Castle Age. Just becomes risky again in Imperial with most Onagers having a range of 9 after Siege Engineers, and Elite Janissaries stay at 8. Similar story for Elite Skirmishers, Janissaries outrange them in Castle Age with 8 vs 7, but in Imperial both become 8. Outranging your counter and having enough damage to kill it quickly is very useful.
This leads me to another question that maybe someone here can answer. Do Condottieri counter Janissaries? They resist the HC bonus damage, but Janissaries don’t deal bonus damage to infantry so maybe they take full damage?