I think that you would first want to have people using both services and annoyances/problems with one service will cause people to abandon the lousy place to use the better place.
That being said, the Lemmy instance I registered to had broken federation approximately half the time and was down for significantly long amounts of time as well. People interacting there had their comments take a long time to federate (only catching up during the rare times federation would work) and they had no idea that they were shouting into a closed box during that time. I’m not even addressing other federation issues such as this instance being blocked by another instance (Beehaw) which is causing some fragmentation.
Lemmy likes to emphasize that you should register for smaller instances and not with larger ones. This “spreads out the load”. You can create your community there as well. You then run into the “annoyances/problems” relating to your smaller instance and migrate to a more stable option… which is Reddit which you still use.
So while federation is a strength for Lemmy, it is also a weakness when it doesn’t work. Migrating people to Lemmy doesn’t tend to focus on migration to a specific server (like lemmy.world ) but focuses instead on migration to “Lemmy” which can be any random server under the sun (stable or not, reliable or overloaded, federating reliably or not). Once issues come up, the person could move to another Lemmy server or they can move back to Reddit… and I think many choose the Reddit option.
It doesn’t help that federation is a complicated topic to understand and the recommended new user approach to Lemmy is to join a tiny server where you are required to use federation and to hope that it is working (while also having no obvious indication if federation is working today or not). To use the email analogy, I get a “bounce back” email notice if my email being sent out cannot be delivered and I get that notice quickly. With broken federation, I have to do research and visiting external sites to determine if my message got through or if I am even receiving any new messages at all. People can get a little annoyed when things are mysteriously not working or when things “may be working or not, who knows?”.