What are the critical parts in a transmitter?

The frequency control portions are most criticall. crystals were used to stabilize frequency and avoid problems with interference or ( harmonics) as it was called. Ham transmitters can be banned if they are off-frequency and produce audible harmonics which can interfere with TV reception. So- for legal reasons, the fine-tuning and Frequency control elements are critical, so are voice or code input jacks, amplifiers. and so on.