Radiant Dawn is frustrating because it has a mix of fundamental issues and structural plot issues, both of which are worth addressing.
The removal of supports is a huge detriment to giving the new characters it introduces a space to develop at a moment where the game feels like it is trying to lean into a fascinating area that no other Fire Emblem games touches by talking about what is, in all but name, a liberal-nationalist revolution (in the style of 1848) through the Dawn Brigade in Part 1.
Part 1 is the only time that the series genuinely takes the spotlight away from royalty (with the exception of the weak and manipulable Pelleas), even if through an often-goofy Fire Emblem-y lens – Path of Radiance comes close, but Elincia is a huge part of that story and Ike is basically a Daein princeling through his relationship with Greil.
Part 2 focuses on internal strife, which is generally where Fire Emblem does its best plots that focus on the royalty – it’s why Lyn and the early parts of Eliwood’s story in Fire Emblem are some of that game’s most compelling parts.
However, this is part of the plot structural issue with the game – it wants to move its camera around and sample a lot of different areas, which means new elements feel undercooked (especially Part 2) and it needs to rely on Path of Radiance to support its cast. It ends up feeling very wishy-washy as it goes into Part 3, partially because it is hard to develop relationships with new units without supports and who are not on screen for most of the story.