Your audience is not in one place. Multistreaming sends the same broadcast to several platforms at once so you do not have to choose.
How it works
Two approaches. Local fan-out encodes once and sends copies to each destination from your machine. A relay service sends one stream to a server that redistributes it.
Local fan-out keeps you in control and costs nothing extra, but it uses your upload bandwidth for each destination. A relay uses one upload, but adds a service in the middle.
The bandwidth question, which people get wrong
Three destinations at 5 Mbps is 15 Mbps of upload, not 5. This is the part that catches people out.
Measure your actual upload — it is usually well below what you pay for — and target about seventy per cent of it across all destinations combined. Leave headroom for everything else on the network.
If the arithmetic does not work, use fewer destinations or a lower bitrate. Prioritise. The platform where your audience actually is deserves the bandwidth.
Encoding once, not three times
Encoding is the expensive part. If your software encodes separately per destination, three destinations cost roughly three times the processing and your machine will struggle.
Single-encode fan-out does the work once and sends the result to each destination. Check which your software does before adding destinations.
When one drops
Platforms have bad days independently. What matters is that a failure on one does not take the others with it, that it reconnects on its own, and that you can see which one is struggling rather than only that something is.
When not to multistream
If your upload cannot carry it, do not. One good stream beats three broken ones.
If your audience is genuinely on one platform, the effort is better spent elsewhere. And if you want comments in one place, understand that multistreaming splits the conversation unless your software merges the chats.