3.5/5. Vivaldi is a straight-forward 2-op FM synth, where you can layer up to 4 parts. Each part is made of very basic components: Three oscillators (carrier, modulator, LFO) and two ADSR envelopes. The envelopes control the level of the oscillators, but the LFO can be sent configured to multiple destinations extensively.
Unfortunately, if you want to emulate classic Yamaha-style FM, this is not the place. I tried to recreate the classic Brass sound, but the envelope or feedback just wasn't providing enough 'umph' on the attack. Even the extremely limited 2-op FM synthesizers Yamaha released are capable of this, as found in various OPL emulation plugins JuceOPLVSTi and vst2413, so it is down to differences to the architecture itself.
Though while not a strict emulation, it does its own thing rather well, and has a pleasant soft FM sound, with still being capable of its own flavor of harsher tones. Definitely more pleasant than other free FM plugins such as Uno FM or mda DX10.
I would recommend Vivaldi MX over the original Vivaldi, as it's essentially a free upgraded version (more polyphony, multitimbral etc.). Also, it's big brother Ganymed is a 3-op FM synth with 2 parts instead of 4.