I think PSG's defense is not that good, but even so, it is very difficult for other teams to compete with PSG. PSG is far above the other teams in the league both in terms of quality and financial power.
PSG started the season badly but over time they started to play better. I criticized PSG a lot at the beginning of the league but now they are playing better football. So I won't criticize them too much.
Nice currently have 35 points and they are 5 points behind PSG. They cannot close this points difference easily. As the league progresses, PSG is less likely to lose points. The players and the team are adapting more and more to each other and they are winning series. I can say that Nice will have a difficult task to follow PSG.
That's one important point about the game of PSG. It is not that important how many goals they concede since they will most of the time score around 2.5 goals per game. It doesn't sound all that much, but the difference is that a team like Nice scores 1 goal on average and if they concede 1 goal the 3 points are gone. PSG can just rely on their offensive game and if it doesn't work out one day, the other teams first have to win their games, which isn't always the case. PSG would need a contender who can win ten consecutive games in order to put them under pressure.