Good team motivation has great benefits if done correctly. What would be a negative side to this?

Maybe someone in the group will act in ego way and force the others to fallow him

the negative site if reflected to malice and hatred between the team in this moment you lose this team

The negative side will appear if the motivator is not true in his/her motivation, and then he will loose all the team trust and will never be able to gain their trust again..
in other words the motivator has to fulfill/prove his motivation prespectives