Synthesizing the DNA itself is not the only problem however.
You then have to get that DNA into a working living form state so that normal cell processes can continue:
  • for viri see: synthetic virus
  • for bacteria, you have to inject it into a cell
  • for placental animals, you also have to somehow simulate a compatible placenta. It is likely easier for eggs.
Basically a synonym for doing a large chunk of de novo DNA synthesis.
Man-made virus!
TODO: if we had cheap de novo DNA synthesis, how hard would it be to bootstrap a virus culture from that?
Is it easy to transfect a cell with the synthesized DNA, and get it to generate full infectious viral particles?
If so, then de novo DNA synthesis would be very similar to 3D printed guns:
It might already be possible to order dissimulated sequences online:
Video 1. 3D Printed Guns Are Easy To Make And Impossible To Stop by VICE News (2018) Source.