Creo currently only allows the mirror constraint to vertices in sketch mode.
Even if we select some arcs, circles and lines, and select the mirror inside sketch, the mirrored geometry per se, does not have mirror constraints, but instead, it's vertices get applied the symmetric constraint. While this is fine if the new geometry is not edited, ie, trimmed, extended, etc, it's rather annoing and time consuming that whenever we later in the process add fillets, chamfers, or trim and extend geometry, the symmetric constraints on the original vertices get deleted by Creo. There was some improvements with the option to fillet and chamfer, but keeping the original symmetric vertices and split the geometry between solid and construction lines, to avoid the annoying deletion of symmetric constraints whenever a simple fillet was added. Nevertheless, this does not solve all the common problems, whenever we need to trim or extend mirrored geometry.
I suggest that the "Symmetric" constraint should be improved to apply the constraint to the geometry lines, circles, arcs, and not to it's vertices. The same as the "Horizontal" or "Verttical" constraint can be either applied to the lines entites, or to its vertices. This would apply a more robust constraint, even if the lines or arcs get edited or trimmed at one of it's vertices.
The "Parallel" constraint is another example of constraint that applies to the entities, and not to the entities' vertices, which only constraints the angle of the line. So, why not apply a "Symmetry" condstraint in sketch mode, tha constraints only the mirroed distance of a line to an axis, or even a point, instead of askying to constraint only it's both vertices to have a mirrored line? When applying the symmetry to a line around a point, constraint the distance of the line to the point. When mirorring a line around an axis (or another line), the constraint should guarantee a symetric angle between the original line, and the mirrored one in relation to the mirrored axis. When mirroring (or applying the "Symetry" constraint between an arc and circle, and a mirror axis, constraint only the center of the circle or arc, and it's dimameter, but leave free the endpoints, in case of an arc, to avoid deleting the symetry constraint if the arc or circle gets trimmed.
Only after thinking about this, did I see that it's precisely this way that the symmetry constraint works for instance in Solidworks. It does indeed make sketching much more productive.