Often, native speakers tell me my use of a pronoun isn't wrong but it doesn't sound good or quite right. I don't have a native ear. How can I tell? For example, I was told that "ones" and "that" don't sound good. But why?

Alternative medicines and treatments take time to work, whereas conventional ones are developed to work fast.

The graph compares the number of store openings with that of closures in one country between 2011 and 2018.

Often, native speakers tell me my use of a pronoun isn't wrong but it doesn't sound good or quite right. I don't have a native ear. How can I tell? For example, I was told that "ones" and "that" don't sound good. But why?

Alternative medicines and treatments take time to work, whereas conventional ones are developed to work fast.

I was told better to use this version:

Alternative medicines and treatments take time to work, whereas conventional medical treatments are developed to work fast.

Another example:

The graph compares the number of store openings with that of closures in one country between 2011 and 2018.

I was told better to repeat "number":

The graph compares the number of store openings with the number of closures in one country between 2011 and 2018.