Some managers just get lucky like Koeman at Southampton. Although they had a strong structure behind the scenes so that probably helped.
Look at his record. Terrible at Benfica, Valencia and AZ. Must be alarm bells there.
Ajax - Good start, then shite, sacked.
Benfica - shite
PSV - Won league on last day on goal difference after being quite far ahead and nearly chucking it away
Valencia - shite, lasted 6 months before getting sacked
AZ - shite, sacked after 6 months
Feyenoord - Didn't win anything but must not have been shite enough to get sacked
Southampton - Done really well with them
Everton - Good first season, then shite, sacked