Stomach issues are a frequently reported side effect of Creatine Monohydrate [1].

The general school of thought behind the belief 'Creatine HCL is easier on the stomach" is:

1) HCL is much more water soluble and (likely) is more easily digested.

2) less creatine HCL stays undigested.

3) the dose needed is smaller.

4) anecdotes seem to support HCL being easier to digest.

To the best of my knowledge, there are no peer reviewed studies of monohydrate vs HCL that look to establish which is easier on the stomach.

[1] https://www.mdpi.com/2075-4663/14/4/137