While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of U.S. Silica Holdings Inc (SLCA 0.94%) fell 3% this morning after Jefferies downgraded the silica miner from buy to hold.

So what: Along with the downgrade, analyst Brad Handler lowered his price target to $31 (from $41), representing about 13% worth of upside to yesterday's close. While value investors might be attracted to the stock's sluggish action in recent months, Handler thinks U.S. Silica's appreciation potential remains limited given its seemingly disappointing execution of late.

Now what: Jefferies lowered its 2014 EPS estimate for U.S. Silica from $2.35 to $1.85, and its 2015 view from $3.00 to $2.30. "It disappoints us to conclude that frac sand demand potential cannot offset execution concerns," noted Handler. "Acknowledging that guidance was intended to be conservative, we find troubling: (1) lack of progress in managing logistics costs, (2) implied lower-than-expected volumes of higher-grade sand and (3) apparent lack of competitive advantage."

When you couple those seemingly real risks with U.S. Silica's hefty debt load, it's tough to disagree with Jefferies' cautious stance.