If the singer can't deliver a good performance of your song, it's not a good song. No one in the audience will say, "It's a good song, but shame about that bad singer." They'll say "Bleh! Let's get out of here."
This means that the path to songwriting success starts with a rather humbling strategy: Make songwriting subordinate to singing.
First of all, you should be willing to transpose a song so that it's in a favorable key for the singer.
If you're lucky enough to have a good, strong singer in the band, one who can take anything you give them and make it sound good, then count it as a blessing. Do what you have to do to keep that singer in the band.
If you're a solo singer-songwriter, working on your singing might be a better use of your time than working on your songwriting. On top of that, you should be willing to ruthlessly rewrite your songs to suit your own vocal strengths and avoid your weaknesses.
If you're writing for another singer, you should know your singer's range (notes they can sing) and tessitura (notes they sing best, usually the middle half of the range). Learn your singer's strengths and weaknesses, and write for them.
If you're writing for an unknown singer, write pessimistically. Assume you'll have an average, barely-competent singer. Stick with a limited melodic range, allow rests between lines for catching a breath, and don't write anything dazzling or acrobatic. Or if you do write anything that requires special vocal skills, don't make the song depend on it. Plan an alternate version that an average singer can perform.
Footnote: Don't be thrown off by what I call "the karaoke effect." You might hear a bad singer torture a classic hit song and still recognize it as a good song. But that works only because the song is already established through repeated listening to its well-performed version. As you listen to the song getting ruined in karaoke, the good version is overlaid on the badly sung version in your mind. But unknown original songs don't have that advantage.
Footnote 2: You'll notice that this blog has a bias towards songs with vocals. Let me just say that I have nothing against instrumental music, or music where the vocals play a less-central role. There are plenty of fine examples in my favorite genre, progressive rock. But in the interests of keeping things simple, I'm going to focus on songs with lyrics and vocals.
Monday is Sobering Truth day at Unruly Beast.