I believe it is extremely dependent on what compound you are talking about, and how complicated the biosynthesis of that compound is.

A simple protein? Very easy.

A complex alkaloid, the biosynthesis of which involves many steps? Super difficult.

I wouldn't call it very easy, it's certainly routine in the lab but still can be a lot of effort. This also has to include purification, not just synthesis. And without a real lab it can get much more difficult. And many proteins are not simple, they can be rather sensitive which makes it difficult to keep them intact while producing and purifying them.

You would probably use E.Coli anyway unless you have to use yeast because of some posttranslational modifications.

Engineering arbitrary modifications is incredibly difficult, due to a myriad of factors, like working against evolutionary optimizations, lack of knowledge of the target organism, reaction pathways that don't go quite like we've sketched them, unspecific enzymes, etc. The success stories you read about are a very small fraction of all attempts. And like another user said, some things are easier than others.