Poch wasn’t a failure for me but there are some who are not satisfied with Poch and his top 4 finishes.
Have a look at the Poch thread in the general football section.
I don't doubt that some people are not satisfied with Pochettino but given that he built a very good team, took us to our highest league position in over 50 years and our first European final for 35 years then they must have an awful opinion of nearly every other manager in the club's history.