Bioreactor is only particularly good for Anglers and Agrarian Idyll (sorta, this one is weak, now), who get buffed farmers. Or livestock builds.
In the short term, it's also ok for catalytic/bioship empires that are going all in on food production techs for other reasons and skipped energy production techs entirely (to focus on e.g. energy weapons). But that's a very marginal thing.
Much hay has been made about whether it would be better to make it also buff technicians (so that they burn food to make energy), or if the building should just turn food directly into energy like its old design did. But either way: the current design is only useful if you have a very strong reason to prefer working farmer jobs.
Otherwise you're better off just working technicians.
why would you NEVER build it? you probably end up with way more food than needed, and the building allows you to skip building ANY energy worlds and have ONLY food worlds.
A farmer gets +60% to food output from tech. A technician gets +60% to energy output from tech. A farmer that also makes energy
does not get +60% to energy production from tech.
So, supposing you have e.g. +90% from other sources (subsidies, designation, stability, etc.), 100 farmers with a Bio-Reactor and Food Processing Center will make 6*2.5=15 food and 2*1.9=3.8 energy. But if you ditched the Bio-Reactor and just used technicians, you could make the same with 15/2.5/8=75 farmers and 3.8/2.5/8=19 technicians (total 94). Same output, 6% fewer pops (and fewer districts needed, in bulk).
If you have more food than you need, convert a food world into an energy world; it will have the same output with fewer pops.
Anglers want to use the Bio-Reactor: many jobs making 8 food, 2 energy, and 2 trade is better than slightly fewer jobs making 10 food and 2 trade plus a few making just 8 energy. Agrarian Idyll is sorta the same (though amenities are cheap, now, so maybe not so much). They are the exception that illustrates the rule: it's only good if your farmer jobs are so much better that you'd rather have more of them.
You can work yourself into that position if you skip all the energy generation tech but did pick up all the food production tech, but that would have to be a rush thing: in the long term you'll be better off just picking up the missing techs.