What happens if you have an engine failure?? Or get
to your destination and you can't get down??

Those are good considerations that would enter into your go/no-go
decision. If you have an instrument rating and the aircraft is IFR
capable, and there isn't ice in the undercast, and you are on flight
following, it is probably not a big deal to get an IFR flight plan on
the fly and descend through the cloud deck in an emergency. Or even not
in an emergency - I have flown many flights VFR above a broken or
overcast layer and gotten a pop-up instrument approach at the end. (I
and the plane were instrument rated). Doing it this way allows a much
more direct routing, and that can cut the need for a fuel stop.

Engines themselves are pretty reliable (though nothing is perfect) -
most engine failures are due to fuel mismanagement. Be extra careful
with fuel if you are planning a trip over an undercast. Ditto for
extended overwater flight.

I also wouldn't think you could navigate by using your map if you
can't see the surface, so that means using VOR or GPS or something...

True. No navigation system does everything.

Jose
--
Get high on gasoline: fly an airplane.
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