The syntax is not great, the standard library is very sparse, and the error messages from the compiler and interpreter are terrible (and used to be worse). The OCaml team used to treat the native code compiler (ocamlopt) as a second-class citizen, but it's the implementation that matters for performance. Since the end of Dennard scaling about 15 years ago, OCaml's lack of multithreading has also been a pain point, one which is finally starting to improve in the last year or two.
I don't think being labeled "functional" was ever a huge stigma. Being labeled "Haskell" is a huge stigma in some circles, but OCaml's never been labeled "Haskell". Until about 10 years ago functional programming didn't become popular enough to have any kind of popular opinion, positive or negative, and the people who knew about it generally thought it would be nice to do more of it.
Mostly I think people use a programming language either because they already know it or it's the scripting language for an environment they have to use. Assembly was the scripting language for your CPU, especially in mainframe days. BASIC was the scripting language for personal computers. Visual Basic was the scripting language for WIMPs. VBA was the scripting language for Excel and Word (previously Excel had a table-based macro thing). sh was the scripting language for the Unix filesystem; C was the scripting language for the Unix system call interface, and a nice upgrade from assembly. JS was and is the scripting language for the browser. Lua is the scripting language for WoW, Roblox, and Minetest. Perl was the scripting language of the WWW, then PHP was, or Ruby if what you really want to script is Rails. SQL is the scripting language for your database. Objective-C was the scripting language for NeXTStep; now Swift is. MATLAB was the scripting language for EISPACK and later *PACK and BLAS, though it had some competition from IDL for a while. Now Python is the scripting language for Numpy (and thus BLAS), Matplotlib, and TensorFlow. Only a few popular languages are exceptions to this rule: Fortran, COBOL, C++, Java, R, Pascal, Golang, and C#.
OCaml? OCaml is the scripting language of ocamlyacc and Coq. If you write OCaml then it's probably because you like Coq. This may have been a public relations problem in the Anglosphere, especially before same-sex marriage.