Every change of manager is a risk. I'm not sure Maresca was a failure, he won the conference league (which the Moyes enthusiasts consider a big success), won the club world cup and was in the CL places when he was sacked. He was doing a decent, if inconsistent job. I'm pretty sure he got the sack because of things unrelated to on field success. Amorim and Frank haven't had that level of success but both Man Utd and Spurs were in bad positions when they took them over, Man Utd in particular look like a basket case of a club, and sometimes things just don't work out. Look at Emery, very successful at Sevilla, had a rough time at Arsenal and then did really well at Villareal and Villa. Does that mean because he didn't succeed one time that he will never succeed again? Of course not, that's a terribly reductive (and frankly nonsensical) argument. Not changing things because they might go wrong is a terrible way to approach anything in life because literally every decision you can make has a chance of not going the way you want it to. The only way to win the lottery is to buy a ticket.