Having worked extensively with MSU data, I'm inclined to
believe it is due to the difficulty of the problem and the
fact that their approach, being the first, wasn't really the
best and thus has needed more additional work.

I agree. I know Spencer, and I've met Christy on several occasions.
They may be global warming skeptics at some level, but they're also
very good and honest scientists. When concerns about methodology have
come up, they've either dispelled those concerns with valid rebuttals
or else tried to address them. I believe their MSU results have been
far more politicized by third parties (right and left) than they have
been by S&C themselves.

There are serious technical difficulties in the use of a series of
different satellite sensors, each with a relatively short lifetime, to
measure small trends over decades.