OK to implement a multi-argument command w/o using braces?

Question

A user on Chemistry SE mentioned a very nice shortcut to use in implementing a command for partial derivatives (see comments section of question: https://chemistry.stackexchange.com/questions/126955/proving-a-thermodynamic-relation-between-partial-h-partial-t-p-and-part):

Suppose you want to create the following partial derivative:

\left(\frac{\partial U}{\partial p}\right)_{\!T}

using this command:

\newcommand{\pd}[3]{\left(\frac{\partial #1}{\partial #2}\right)_{\!#3}}

The standard use of the command would be:

\pd {U}{p}{T}

However, since each argument is only a single character, this is sufficient:

\pd UpT

If one needs to typeset numerous partial derivatives, this is a great time-saver.

Indeed, you can even use a hybrid approach, where braces are eschewed for single-character arguments, but used for multi-character arguments. For example, to implement this:

\left(\frac{\partial U}{\partial p}\right)_{\!T, n_i}

One would write this:

\pd Up{T, n_i}

This seems to leverage what is a standard shortcut in LaTeX for single-argument commands (e.g., we drop the braces when the argument of a superscript or subscript command has only one character). But, for this usage (multi-argument user-defined commands) are there cases in which this (seemingly non-standard) approach would create a problem?