@0hlov3 from dendrite's readme: "Dendrite [...] intends to provide an efficient, reliable and scalable alternative to Synapse". From what I hear this holds true, especially the memory consumption is much better.
The disadvantages are currently missing features from the matrix spec. As they list themselves, most notably: search (channels/spaces), user directory, presence. Additionally I wouldn't expect application services to fully work yet (they don't explicitly mention it though).
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