Hi, first off let me say I am a Intermidiate/Advanced beginner on sewing machines.
Next I have noticed uneven stitches when I sewd on thick wool with Coats and Clarks Mercerized Cotton thread (I think it is old thread) and a Schemetz denim needle.
Furthermore, I noticed uneven stitches when I sewd on thin poly/cotton(?) fabric with the same thread and a Schemetz 70/10 needle.
The last instance I want to mention is uneven by turning into little stiches going over a thick seam (which may be normal, idk).
Also I should mention as I'm still a beginner my speed isn't consistent. As an example, sometimes I speed up, then slow down quickly.