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.