Local continuity vs. Fairness:
take these two crv. notice the bad cv placement in the left one.
it causes the blend crv to dip and will result in a dent in the surface.
–The resulting crv has perfect local continuity but poor fairness.
matching only to g1 tangency aviods this and depending on the specific geometry can give good results. Also “your method of production” can hide the small tangency break. I would argue this resulting crv has better fairness at the cost of continuity.
the proper solution is to trim the crv back and blend over a larger distance.
"bridging" crv over the centeraxis:
the top curves describing both sides with one singlespan crv , in contrast to the ones below which take two. the top one has the benefit of being always smooth over the center while the other one needs to be matched. notice the “g3 break” after transforming in the lower one.
sadly I don’t know of any native tool in Rhino to enforce symmetry over a axis - so this tends to be manual labor