It seems that flying a plane in a slip can make passengers uncomfortable.
What is it about that configuration that is unsettling?
A slip is uncoordinated flight and this shifts the effective gravity vector you experience. Passengers in the back are also at a disadvantage without a visual horizon in front of them. This can induce a mismatch between what the passengers can see versus what they are physically experiencing. In some people this can cause discomfort and potentially vertigo.
Other discomfort can be psychological in nature that the airplane isn't behaving how they expect. Passengers expect if a plane isn't turning that the wings are level. If you are flying when the plane lands in a crosswind with a slight bank and touching down with one main gear, then the other main gear, many passengers will comment that the landing was incorrect because the main gear didn't all touch down together and the pilot was flying "sideways". They don't realize the wind requires this technique and that the landing was in fact correct. The perception of things not being right, however, can cause discomfort in some passengers.
Slipping a plane is a form of uncoordinated flight during such a case the plane is inherently unstable and more susceptible to turbulence as caused by unstable air.
Some of the advantages of coordinated flight are
Coordinated flight is usually preferred over uncoordinated flight for the following reasons:
Comfort level of the passengers (the seat of their pants won’t be going this way and that).
Drag is kept to a minimum, helping to maximize the margin between actual air speed and stall speed.
Fuel will continue to be drawn equally from tanks in both wings, especially important after a long flight when fuel level may be low (of course you planned your flight to land with legal reserves, right?).
No cross controlling, no stalls, therefore no spins (bad, very bad).
To answer the question directly its the bumps that are unsettling however this is dependent on the situation. In calm air you, as a passenger would never know (but in calm air you really don't need to slip a plane). In a small GA plane where you can better see whats going on it may appear confusing on final approach the pilot is using heavy control inputs and the plane is moving straight (towards the runway that is)