still she could have stolen it and eat it while Kaido is not there and since she laid low no one would assume that she is the one , she were waiting for Luffy for years never take part in Kaido's forces and it seems that even Sasaki didn't know she has a DF.
When will you learn that its situational? Zoan forms are the most durable and most often have the highest raw strength their are instanceslike luccis where his animal form is faster but hybrid is bulkier. in the case of the dinos and kaido, they have the most raw power, are at their durablest and often most destructive in their full zoan, but they are big, clumzy and slower. Take dragon form kaido, put it against an army and it would wipe them muuch faster than hybrid. But as soon as someone is good enough to avoid its attacks, it looks clumzy and slow and gets ragdolled and humiliated. When the forms are at their strongest depends solely on the situation and opponents.