Firstly, the main disadvantage of using DNA directly is because DNA and protein synthesis needs to occur at different cellular locations, without which DNA would have to be in the same environment as ribosomes making DNA more vulnerable and exposed.
Secondly, DNA contains a wide range of non-coding regions to regulate the replication and transcription process, which means using DNA as a direct template would require the elimination of these regions. It would impair the regulatory capability of eukaryotic DNAs.
Thirdly, the splicing process creates an additional layer of possible diversity when it comes to the splicing of introns and exons. A single coding region of DNA can create variations of the protein by shuffling different exons and introns, which can lead to translations of several proteins from one region.