Underwriting in terms of tightening up "songcraft" and Overwriting by being too un-creatively "obvious". This could, perhaps, apply to all songwriters writing for all types of listeners. But this dual malaise is particularly represented in a lot modern/post-modern songwriting intended for a Christian audience, one I'm fairly familiar with.
But there seems to be a marked difference between "I jettisoned using a rhyme scheme for artistic/aesthetic purposes" and "I wouldn't know a rhyme scheme if it bit me on the heinie." Skillfully attending to the craft of songwriting, in terms of listeners' awareness, is often a thankless task. If you do your job correctly, no one but you will have to think about it too much. But whether the listener knows it or not, a well-designed song means less distractions and a more effective delivery of the lyric or enjoyment of other elements of the song (music, groove, great production, etc.). Again, there's nothing hard-and-fast about all this.
One of my songwriting heroes insists on perfect rhymes (bring and thing as opposed to world and girl). So when I put on my ruthless editor hat, I call it "Getting in touch with my Inner Jimmy Webb!" His book Tunesmith is still, to my mind, essential reading. Part memoir, part primer, part manifesto.
"Overwriting" is the mistake of removing any sense of mystery or discovery in your lyric. Two related images come to mind: reaching over to cut up the steak on someone else's plate … and (pardon the unsavory nature) pre-chewing someone else's food. This is particularly egregious in the most well-intentioned Christian-themed songs. Of course in any songwriting, there are choices a writer makes as to what will be shown and what will remain hidden. To mix images even further, it's very much the songwriting equivalent of a newspaper editor managing the news.
But to rob a listener of the sense of discovery, of finding their own place in a song. This results in a sometimes unintentional artistic theft that leaves almost everybody unsatisfied. Or, God help us, leaves some people satisfied with zero effort to actually engage with what they're listening to. The example I've cited for years on this is Paul Simon's masterful lyric "Something So Right".
They got a wall in China
It’s a thousand miles long
To keep out the foreigners
They made it strong
And I got a wall around me
That you can’t even see
It took a little time
To get next to me
A lesser writer would have settled for the most direct expression of the thought "I'm a little hard to get to know" and that would've been that. But Simon weds that simple notion to the Great Wall of China. Creative and unforgettable.
Yes, sometimes you need to be Captain Obvious. But for my money, more often than not, the most efficient route to communicate an idea is not the most direct route. When the listener has room to maneuver and discover, they can make your song part of their soundtrack. You give them credit for being at least as smart and intuitive as you are.