The establishment of structure-function relationships requires the accurate characterization of the materials being studied. For polymer science, size-exclusion chromatography (SEC) is among one of the most relied upon tools. Herein, we investigate the use of SEC for the characterization of structural parameters, composition, and purity for bottlebrush polymers (BBs). The investigation comes in three parts: the first section focuses on determining the five independent structural BB parameters. The second section provides a systematic investigation into the use of SEC of determining the composition and purity of diblock BBs. The third section considers the sensitivity of SEC to processing variations. This manuscript endeavors to raise awareness of the challenges associated with SEC analysis and provides practical information to improve the rigor of polymer characterization to aid in the development of accurate structure-function relationships.