The old "half of my advertising budget is wasted, but I have no idea which half" quote springs to mind...
Not sure how you're defining "underfunded" candidates, but the incumbents in competitive races where enormous amounts are spent by both sides to try to gain an advantage don't win anywhere near as often as the incumbent in nom-competitive races which parties and PACs barely bother spending in. Ultimately the spending is positional and cancels out, and the biggest spender often loses because there's not nearly enough difference between the candidates' spending levels to affect whether voters hear their messages.
Not sure how you're defining "underfunded" candidates, but the incumbents in competitive races where enormous amounts are spent by both sides to try to gain an advantage don't win anywhere near as often as the incumbent in nom-competitive races which parties and PACs barely bother spending in. Ultimately the spending is positional and cancels out, and the biggest spender often loses because there's not nearly enough difference between the candidates' spending levels to affect whether voters hear their messages.