Weaknesses of Momentum Investing

Common investors usually do not pick up the trend at early stages. They join when most of the potential gains have been apportioned by either the financially expert or the institutional investors.
Sometimes, the trend reversals may be very sharp, and may erode all the gains made by investors entering late. Also, recoveries from retracements may not be good enough to give the investors chance to wind up profitably.
The investment horizon of momentum investing is usually short. So, short term capital gains tax can make the strategy less attractive.
Going long in a stock based upon momentum, which otherwise is not fundamentally sound, aggravates the market inefficiency.
Market needs to be watched continuously for catching early trend reversal signs. So it needs more effort by the investor to be safe.