Gary is correct that, for songs in major key, I (the tonic major chord) is by far the most common ending chord in Western music in the last several hundred years. Most commonly, a V chord precedes it, because the V chord contains the notes 5-7-2 of the scale, and both the 7 and 2 notes lead the ear to expect resolution to "1," the root note and chord. Of course many exceptions to both of these generalities will come to mind, to those who know enough songs overall.

A song with flat VI and VII can be thought of in a couple of ways. One way is that it's a song in minor key, but with prominent Picardy (sharpened) thirds. It's not at all surprising for a song in the key of Em to have C and D chords. It's surprising, though, to change that Em into E major. Think of the ending of "Coventry Carol." Obviously the surprise lessens on repeated occurrences. A recent pop song which uses this effect regularly is Pharrell Williams' hit "Happy" from 2014. It has numerous Esus resolving to E major, although the prominent G and D notes in the melody and accompaniment lead the modern ear to hear the song as in Em overall.

https://www.youtube.com/watch?v=y6Sxv-sUYtM

Another way to think of a song in the key of E with prominent C and D notes (or chords) is that it's in the Phrygian mode, whose notes are EFGABCD, with a tonic (or final) of E. With a final of C, that would be C-Db-Eb-F-G-Ab-Bb. A good example of this is "Why Fumeth in Fight," a choral setting of Psalm 2 by Thomas Tallis, the royal composer of the house of Tudor from Henry VIII through Elizabeth. To those of us used to heavy use of standard major and minor scales, its ending seems "unfinished" since the "final" in the Phrygian scale sounds to us like a III chord, which would be a strange place to end a song.

https://www.youtube.com/watch?v=lD5TG8z3-SM