one-trick pony
US /wʌn-trɪk ˈpoʊ.ni/
UK /wʌn-trɪk ˈpoʊ.ni/
1.
a person or thing that has only one special ability, skill, or area of expertise
:
•
He's a talented singer, but he's a bit of a one-trick pony, only performing ballads.
•
The new software is a one-trick pony; it only does one thing well.