I only run/maintain synapse servers right now 
I also kinda love how synapse still has
in its README 
But it is the only spec-complete implementation as of right now.
Both Dendrite (Matrix/Element people) and Conduit (famedly) are still lacking some Server2Server and Client2Server APIs.
→ dendrite/README.md at d983d17355584b7de086389e069a935f1a5510a1 · matrix-org/dendrite · GitHub
→ Issues · matrix-org/dendrite · GitHub
→ README.md · a2a7c6187228bbcbbefadaa703e5afe469b7cbfe · Famedly / Conduit · GitLab
→ Conduit 1.0 · Milestones · Famedly / Conduit · GitLab
Dendrite is becoming more and more complete, but it will still take a while before it is truly complete.
All the cool p2p demos are built on dendrite though (Matrix.org - This Week in Matrix 2022-03-18) and (Redirect) and (https://matrix.org/blog/2021/05/06/introducing-the-pinecone-overlay-network).
I can’t really speak for Conduit because I don’t follow its development as much.
Oh, and most experimental MSCs [Matrix Spec Changes] usually land in synapse first, if you care about that.
So I would argue, stick with synapse for your main matrix.org server for now and maybe run another implementation on a separate (sub-) domain 