Franck Lampard is a legend in Chelsea and even during the time when he was a player in this team he had a good performance also as a coach is suitable for his team because they can't have enough time to hire another coach, however, it was a big risk for Lampard to work in Chelsea because they don't have a good situation there is a possibility for them to even fail with their coach, Frank Lampard.
Chelsea may have no choice now, they are in hurry to hire a new coach. Frank Lampard has a close relationship with Chelsea because he is a Chelsea player, that's why Frank Lampard is appointed as an interim coach. I agree that bringing back Lampard to Chelsea is a big risk, he ever managed Chelsea but it ended with an unsatisfying result. Lampard may end this season with the same result as previously (bad result). I assume that Chelsea doesn't target anything, so they hire Lampard although this man has no good record as a coach.