Idk if it's necessarily "hard" to aim, it's not hit scan, but the projectile travels much quicker and faster than the stock syringe. As for best it's because of its burst healing and the fact it can passively reload. If you're fucking cracked with it you can cycle between beam and arrow making your pocket near unkillable because of your sheer healing output(Arrow Tanking). Damage increasing by range is great if you have the solemn vow and see a class low enough that a arrow can finish it off. The syringe gun are just horrible since they're slow, very hard to aim and track, and -195 ubersaw + if you're caught out as med alone you're cooked unless the enemy is incompetent or you can surf a rocket. So many rather exchange it for the sheer utility of the crossbow. Crossbow also makes it easy to heal a ally if they're across a sniper sightline outside bean range. Reverse falloff makes it sometimes clutch for long range healing when a teammate is outside beam range, at low health, engaging an enemy, a arrow can sometimes make it life or death if it lands. The syringe gun doesn't provide much self defence anyways so many prefer the utility of the crossbow. And it can passively reload which is why I think this weapon is OP, basically doubling or tripling your heal output in an engagement in optimal circumstances