uic considers that the Universe starts with the run button, there is no prior knowledge or history, you are God and you are wielding time, so the solver is calculating everything from scratch, which can take a long time if the settling times need to.
Without it it's what @glen_geek said: it tries to solve the circuit for DC since it considers that the Universe has started long before, there is plenty of knowledge from the history, so the circuit has had time to settle into what are today's values. When it does so, it does it with what it sees in the circuit, not with what it reads from our minds, since it cannot do that. This is up to us to set in the circuit.
start supplies from zero (or something like that) only adds a minor startup for supplies, so the circuit is solved for initial conditions, but with the difference that it still tries to solve for DC, since it's not
[in reply to the comment]
As I said, the solver cannot read minds, in can only read circuits. In this case, it sees a supply with no series resistance but a parallel cap (useless, internal resistance of voltage source is zero), a FET driven directly by a voltage source, with 3.3V, no resistor, rectifying diodes that are too slow for the switching frequency, a filtering cap with no series resistance, and a load that may as well be open air. For these, the solver found a solution.
uic forces a different starting solution, most often the real one, but takes time to settle.
If you probe in different parts, you'll see that
uic and normal give different starting points for every voltage and currents. For example,
V(out) starts from
4.1V, not zero, as you'd expect. That is because, as I said, without
uic, the solver considers that the circuit has been there for ages, in steady-state, no switching, so everything is calculated based on static voltages and currents. And you got ...some initial solution. With
uic, everything starts from zero, DC, switching, everything. Which means that the solution will be radically different. So you cannot blame the solver for trying to accomodate your very unrealistic (and quite poorly made) circuit, that is, you shouldn't blame the tool for how what user makes of it.