Potentially a good move for him. I felt a move for either of A.Madrid/Sevilla would have been good for him. A.Madrid for the opportunity to work with Diego Simeone, as well as the persistent injuries of Giménez. However, Nehuén Pérez might be returning on loan. Sevilla as a replacement for Diego Carlos who was linked to Barcelona amongst others.
Salisu went from playing a semi-professional division in Spain, to playing in La Liga which is a big jump. And now he'll have an even bigger jump playing on the European stage with Rennes (should they succeed in playoffs).
With Rennes finishing 3rd, they would be in the Champions League play offs under normal circumstances. However, Corona complicates things. Uefa Executive Committee are having a conference meeting via video on June 17 and 18th which will address the qualifying phase for 2020/21 UCL. It would be a shame for Salisu, if it was established that Rennes will be one of the sides to miss out on the Champions League. According to newspaper, The Times, UEFA might explore the option to limit/shorten the playoffs to those that won their respective domestic leagues and then financially compensate the rest. I reckon Rennes will be fine.
Provided he continues on his trajectory, at International level he could potentially form an interesting defensive triumvirate - Salisu, the upcoming Nathaniel Adjei, and Partey.