I don't think you are underestimating PSG, but the way you are judging them makes it seem like Inter are unbeatable. Remember, this is the final match, PSG strength is also not as weak as imagined. In addition, Inter is competing with Napoli in the hunt for the Scudetto, in conditions like this, Inter players are very vulnerable to injury if Inzhagi continues to force them to play full time in every match.
Inter Milan and PSG are nearly the same in quality but PSG will have more advantage than Inter Milan in their preparation for the final match. In Ligue 1, they already got the title and while in Serie A, Inter Milan will have to fight in their last two matches because of having one point less than Napoli now. I believe that Inter Milan won't give up in Serie A and this will cost them somewhat energy and worsen their preparation for Champions League final match. This can be a main disadvantage of Inter Milan which can be exploited by PSG players who will use all energy and the best things they have for this final match.
Luis Enrique is also trying to become the first coach to give the UCL Trophy to PSG, besides he also wants to follow Guardiola who managed to achieve the Treble Winner with different clubs. Currently the chances of both teams are still 50:50.
Both coaches will try to win this final and Enrique chasing a first Champions League title for PSG but Inzaghi looking for a first Champions League title in his career as he has never won any title as either a player or a coach. Enrique already won several Champions League titles with Barcelona as a player and a head coach.