I suppose it depends on your definition of "adequate." The Bonanza I fly
has overhead map lights that do an excellent job lighting up the cabin, but
I do not to use them to brief or follow an approach plate due to the night
vision damage these lights cause.

And in my opinion, that makes them inadequate - for all the reasons Roy
mentioned. There is no reason those map lights should not have a red
filter. Ot two bulbs - red and bright - switch selectable. If you
have to juggle flashlights, turn your head to look at a plate rather
than just glance (vertigo city, IMO), and otherwise compensate for a
lighting system that doesn't actually allow you to see at night, then
your workload goes up - and single pilot IFR in IMC workload is already
high. Can you do it? Probably, but can you do it consistently 100
times out of 100?

It's OK to have an emergency procedure that only gives a 99% success
rate. Odds are you will never use it, so the overall odds of using it
an having it fail are tiny. I don't think it's OK to have a normal
procedure that only works 99% of the time. If you fly any significant
amount of night-IMC, it's going to bite you.

That is why I consider no vacuum backup acceptable, but a lighting
system that requires juggling flashlights unacceptable. How many
vacuum pump failures have you seen? I've seen two in 1500+ hours of
dry pump operation (wet pumps pretty much don't fail). Thus between
the low likelihood of the event, and the high likelihood (for a
proficient pilot) of handling the event, it's no big deal. The
flashlight juggling happens on EVERY night flight you make in an
improperly equipped airplane. Now what's harder - partial panel IFR in
day-IMC (or night-IMC in well lit cabin), or full panel IFR in
night-IMC in an airplane where you have to juggle flashlights?

Michael