I think Fonseca is a very good manager but he found himself in a bad situation at Roma.
Implementing his style of play in Serie A was going to be hard to do anyway, remember those Champions League Shakthar teams that had great passing, width and fullbacks pushing up, but with Roma's financial situation, injuries and ownership changes he was never going to have time to settle.
He clashed with Dezko and stripped him of captaincy but overall players liked him, I remember Smalling and Pellegrini in interviews being very positive about him even after he was let go.
One thing fans disliked about him was that he had a terrible record against big teams in Serie A, but that's hard to blame him for because Roma just wasn't on their level talent wise.
Second con against him was his inability to adjust tactically in game, doesn't matter if it's against Inter or Benevento once other team switched up and took over the match you just have to hope to survive last 20 minutes.
But I think he will be a great manager if he lands at a right club, objectively he did the best he could with Roma.