Modern 3D printing can be categorized into several technologies like FDM (Fused deposition modeling), SLA (Stereolithography apparatus), Polyjet, SLM (Selective laser melting) and BJ (Binder jetting).

• FDM is all about cost-effectiveness. Good for polymer antennas, but not the best for high frequencies.
• SLA is about accuracy. Good for ceramic antennas, although speed and size can be a trade-off.
• Polyjet: Multiple materials, multiple possibilities.
• SLM: Directly prints metallic antennas. However, accuracy might not cut it for high-frequency needs.
• BJ: Limited accuracy, but low cost and high speed of manufacturing. Can print metals.

As you can assume, the biggest challenge is accuracy. Moreover, most popular 3D printing technologies cannot print metal (or multi-material), thus they require post-processing metallization.