Those who have kept up with Saturday Night Live over the years will recall any number of comedic skits that worked very well as two- or three-minute shorts on the show, but when expanded into full-length feature films were tedious and ineffective.
That's a good comparison for McCartney's oratorio here. There are passages that are quite effective, and nicely done; here and there, for thirty seconds or so at a time, you think hey, this isn't bad. The problem is that this piece lasts a full hour, and like those SNL skits it just doesn't hold up when expanded to these proportions. There's no formal or structural integrity, no large-scale dramatic rise and fall, or ebb and flow. Rather, we have the simple song forms with which McCartney is familiar (AABA, etc.) expanded ad absurdum (AAAAAAABBBBAAAAAA, etc.).
One of my convictions about lengthy musical works is that they must justify their own duration. There has to be a really good and self-evident reason for piece of music to last twenty, forty, or sixty minutes, for it is entirely possible to present a thoroughly satisfying musical experience (to present multiple musical ideas, develop them, bring a sense of resolution towards the end, and close things out) inside of three or four minutes. This can be seen in the best popular songwriting, in a great deal of jazz improvisation, in classical miniatures such as the Chopin etudes and nocturnes for piano, etc. Ecce Cor Meum wholly fails on this count, then: there is simply no good reason for it to last so long, no skillful development of ideas or meaningful build and then release of dramatic tension.
Furthermore, while there are indeed some effective passages here, a great deal of the writing is also flat and artless. McCartney does not and cannot write counterpoint, which is the essence of choral music; instead, Ecce Cor Meum utilizes a sort of a lead singer/backup singers construction, like what you'd expect to find in a pop song. Occasionally there's a little call and response, a few instances of two- or three-part voice leading, but otherwise the choral and orchestral textures are simplistic and empty. It's such a wasted opportunity to gather a choir of hundreds of voices, as McCartney does here, only to have them all singing pretty much the same thing at the same time.
I really don't mean to be too harsh. There's nothing WRONG with this music. It isn't unpleasant. But in tackling a form like the oratorio, McCartney is placing himself in direct comparison to history's master composers and great musical geniuses, and unfortunately he falls far, far short of them. For all its pleasant moments this is vapid, uneventful music.