Some Schirmer editions are good and some are not so good. As Richard Black said, they are inconsistent.
The Schirmer edition of the Liszt
Annees de Pelerinage (Italie) edited by Rafael Joseffy, for example, is excellent because Joseffy worked very closely with Liszt himself on that edition. Similarly, Schirmer's Sauer Edition of the Brahms
Short Pieces is excellent because Emil von Sauer was not only an editor in this instance, but a wonderful artist as well. He had even played Brahms' works in Liszt's master classes, fully knowing that Liszt disliked Brahms! Schirmer editions like these continue to serve pianists well.
In contrast, Schirmer's Czerny edition of Bach's
Well-Tempered Clavier, is over edited, very likely not historically accurate, and long out of vogue. (Czerny claimed he heard his teacher, Beethoven, play all 24 Preludes & Fugues and recalled it in detail

). Generally speaking, pianists would best use urtext editions such as Henle, or similar scholarly undertakings (like the fine and highly respected Paderewski editions of Chopin). Again, some Schirmer are very good, depending on the editor.