Do they also question it’s rightful place in Scripture, or simply its authorship?
There are six of the Pauline epistles which have been questioned by some scholars--including Ephesians, Colossians, and the three "Pastoral Epistles" (I & II Timothy and Titus). The arguments have to do with style and the idea that some concepts are more developed than in the definite letters of Paul. The stylistic differences might be dealt with if they were written by a secretary writing under Paul's direction, but otherwise they could be by a later disciple who sought to develop Paul's ideas for a later period (A.D. 80-100).
The New American Bible for Catholics has introductions to each book and mentions these scholarly arguments (without taking a definite stand)--but not for II Thessalonians. But the introduction to Philippians mentions the theory that the letter as we have it is a composite of three letters by Paul to the Philippians.