The book Burt Rutan's Race to Space by Dan Linehan describes why the predecessor of Spaceship Two chose a hybrid engine.
Rutan ruled out solid motors because they cannot easily be shut off (a safety concern).
He ruled out liquid motors because of cost and complexity.
Rutan settled on a hybrid engine as the best compromise between safety and cost. Also
CubeSat propulsion systems employing HAN-based propellants are currently under development and slated for launch in the next couple years. AF-M315E, the most developed HAN-based propellant, has very low shock and impact sensitivities. In light of this (along with the negligible vapor toxicity), range safety management has designated AF-M315E in a lower ...
According to this nasaspaceflight.com forum post, the dynamic viscosity of Falcon 9 chilled RP-1 is about 3.3 cP.
The table here lists the viscosities of some maybe-not-so-everyday fluids; Falcon fuel is kept twice as viscous as ordinary kerosene at room temperature (1.64 cP), comparable to milk or blood. Water at room temperature is 0.89 cP.
I tried to ...
In addition to the "rough combustion" that Organic Marble mentions, liquid engines also characteristically suffer from pogo oscillation. This is the phenomenon in which the thrust causes acceleration of the rocket, which changes the flow of propellant in the lines fuel and oxidizer lines, which then causes a change in the thrust, which is now a loop.
Yes, in liquid engines this phenomenon is called "rough combustion".
Sutton (4th edition) says
Combustion that gives pressure fluctuations greater than about +/- 5%
of the mean pressure at a chamber wall location and which occur at
completely random intervals is called rough combustion.
It's different from "combustion instability" defined as ...