Sarabha is a form of mythical bird which is a mixture of lion, man, eagle. That is this form is a bit greater than a combo of man and lion (narasimha). Since the anger of narasimha becomes uncontrollable, shiva took this form and pacified him. There is no difference between narasimha and saraba. Sarabeswarar himself said that they both are mixed like ghee in ghee and milk in milk.
The major thing to note here the two wives of saraba are Saraba soolini (Shri durga) and Prathyangira. These 2 forms of sakthi turned as two wings of saraba and then these two wings only controlled narasimha. Because, the male form alone will be filled with anger and uncontrollable. (Even the sarabha without his shakthi turns uncontrollable). SImilarly lakshmi narasimha is also very santha in nature. Since these two powerful sakthis are there, while we worship him, we will get the grace of 4 big deities (Narasimha, sarabha, durga, prathyangira). This is why the power of sarabha is this much great.