Are there any major side effects of stent placement?

Rarely: Once properly placed, major side effects are rare. It is possible to get scarring in a stent (in-stent restenosis). It is also possible to get stent fractures (a break in a stent). These take time to develop if at all. Infections of the stent are rare. People rarely develop allergic reactions.

Yes: I assume you mean coronary stent. During placement there is risk of bleeding or heart attack. The contrast dye can harm kidneys. Afterward, there is risk of clotting the stent causing heart attack. The blood thinners needed afterward has bleeding risk. However, the risk must be compared to heart surgery or heart failure or death. Tech/training has changed greatly in recent years so is safer now.

Complications: Vessel damage or rupture. Bleeding, and infection. Contrast reaction.