I used a poncho that came down well below the knees. I also wore shorts, not trousers (I walked in May/June). That's relevant because in steady rain, water will drip down the poncho and onto your legs. That's no big deal with shorts, but might be a hassle wearing trousers.
I liked the poncho because it was very light and could be carried in a pocket and donned quickly when rain came. It was also voluminous enough to cover the backpack.
One minor irritant was that the wet poncho clung to the legs and had to be shaken free occasionally to stop too much water running into the shoes. Shoes will get wet anyway, and it is no big deal. All the albergues have heaps of newspaper to stuff them with for drying. Newspaper overnight is amazingly effective for drying shoes.
However, having seen the pic of the
ALTUS jacket, that looks like a great alternative.
I also took light plastic trousers and jacket, mainly as a wind break to keep warm when crossing high passes.
I wore the jacket one day when the rain was heavy for some hours and penetrated the poncho. However, the plastic jacket did not breathe and I soon became wet (and smelly!) anyway from sweating under the jacket.
Re ponchos (and all rain gear for that matter), look for gear that has the fewest leak points such as seams that are stitched but not waterproofed. Complicated, insulated jackets with pockets, flaps and assorted attachments may also leak.
Hope that helps. I have posted other stuff on the forum re equipment in the past. Search on "bobm" and you will find it.
Bob M