Why are thrusters so difficult?

Excuse me, could you elaborate on why thrusters are considered so challenging to design and operate? Are there specific technical hurdles that engineers face when developing these propulsion systems? Is it the intricate balancing of forces, the extreme operating conditions, or the need for precision control that makes them particularly difficult to master? Understanding the complexities involved in thruster technology would greatly enrich our discussion on the advancements and limitations of spacecraft propulsion.