With regard to:
"Q: Are your statistics reliable?
A: No. As said before, given the enormous work ahead of us, our site is far from comprehensive and doesn't claim to be. Also, the entries are heavily biased by the preferences of the editors and our visitors. Conclusion: Never ever claim Summertime] is the most covered song with more than 750 covers according to SecondHandSongs."
Has anyone at SHS ever done rough numbers to see what percentage of "modern" "hits" (20th century popular music) are possibly represented so far (clearly not all originals that have charted / sold have covers)?
Since no one else is even trying to keep track so thoroughly, it's fair to say, for what's known, SHS certainly is reliable. Complete? Obviously, no. But, the depth of care for citation and "shepardizing" is beyond anything any other repository of knowledge is even attempting, let alone achieving.
Just 2 cents from the peanut gallery.