There are a number of drawbacks to share repurchases. Both shareholders and the companies that are repurchasing the shares can be negatively affected.
Shares may be repurchased if the management of the company feels that the company's stock is undervalued in the market. It repurchases the shares with the intention of selling them once the market price of the shares increase to accurately reflect their true value. Not every shareholder, however, has a fair shot at knowing whether the repurchase price is fair. The repurchasing of the shares benefits the non-selling shareholders and extracts value from shareholders who sell. This gives insiders an advantage because they are more likely to know whether they should sell their shares to the company .
Martha Stewart
Martha Stewart was convicted of insider trading, which is not the same as insiders choosing whether to sell their shares in a share repurchase. Insiders are still at an advantage because they will know not to sell during the share repurchase.
Furthermore, share repurchases can be used to manipulate financial metrics. All financial ratios that include the number of shares outstanding (notably earnings per share, or EPS) will be affected by share repurchases. Since compensation may be tied to reaching a high enough EPS number, there is an incentive for executives and management to try to boost EPS by repurchasing shares. Inaccurate EPS numbers are not good for investors because they imply a degree of financial health that may not exist.
From the investor's standpoint, one drawback of share repurchases is that it's hard to judge how it will affect the valuation of the company. Companies often announce repurchases and then fail to complete them, but repurchase completion rates increased after companies were forced to retroactively disclose their repurchase activity. It is difficult for shareholders, especially relatively uninformed ones, to judge how the announcement will affect the value of their holdings if there is no guarantee that the full announced repurchase will occur.