Pasted from Wiki:

If two or more teams are equal on points on completion of the group matches, the following tie-breaking criteria are applied:[1]
  1. Higher number of points obtained in the matches played between the teams in question;
  2. Superior goal difference resulting from the matches played between the teams in question;
  3. Higher number of goals scored in the matches played between the teams in question;
  4. If, after having applied criteria 1 to 3, teams still have an equal ranking, criteria 1 to 3 are reapplied exclusively to the matches between the teams who are still level to determine their final rankings.[a] If this procedure does not lead to a decision, criteria 5 to 10 apply;
  5. Superior goal difference in all group matches;
  6. Higher number of goals scored in all group matches;
  7. Higher number of wins in all group matches;[b]
  8. If on the last round of the group stage, two teams are facing each other and each has the same number of points, as well as the same number of goals scored and conceded, and the score finishes level in their match, their ranking is determined by a penalty shoot-out. (This criterion is not used if more than two teams have the same number of points.);
  9. Lower disciplinary points total in all group matches (1 point for a single yellow card, 3 points for a red card as a consequence of two yellow cards, 3 points for a direct red card, 4 points for a yellow card followed by a direct red card);
  10. Higher position in the European Qualifiers overall ranking.

  1. ^ If there is a three-way tie on points, the application of the first three criteria may only break the tie for one of the teams, leaving the other two teams still tied. In this case, the tiebreaking procedure is resumed, from the beginning, for the two teams that are still tied.
  2. ^ This criterion could only break a tie if a point deduction were to occur, as multiple teams in the same group cannot otherwise be tied on points but have a different number of wins.
Sourced from UEFA's competition rules:

I can see the virtue of potentially giving the last game more incentive and jeopardy by enabling a team that would otherwise be way behind on GD to overcome a side that is 3 points ahead going into the game. OTOH there's more incentive to defend a 1-0 than go for the GD.

Are there other drawbacks I have overlooked?