Currently, after playing 3 or 4 games with death magic mixed in as well as life magic, its seems not just weak but focused in the wrong direction for making it unique.
The elemental spell categories do pretty well within the expected specializations: fire is mostly damage, earth is protection and growth etc etc.
Life magic seems (with heal especially) to lack scaling but at least be relevant as well as thematically correct. Death magic on the other hand just doesn't really seem a worthwhile choice. It follows thematic elements (debuffs, sacrifice for power etc) but just lacks distinction in effects and falls down in relative strength.
My suggestion is also something of a question: where is the branch of magic that heavily invests in summoning? Every stock fantasy cliche of a evil wizard (outside of the LOTR perhaps) figures heavily in summoning or raising troops from somewhere else. Necromancy and Demonology figure rather prominently in the idea of the bad guys but in FE you can't summon so much as a skeleton. The elementals not withstanding, summoning troops just isn't much of a part of the game.
In comparison to its inspiration (MOM) this seems a rather large oversight: creatures that could be summoned were a very large part of the game in every branch of magic. Even certain global spells emphasized summoning (zombie Mastery anyone?).
My suggestion would be that with Death magic currently being a poor cousin to the other branches, and the lack of summoning creatures in general would be to re-orient the death magic category to more of a summoning branch of magic than an effects one. Bring back the armies of the shambling dead!