Sentinels are basically an attempt to create the 'pure ranged with an armor' archetype, but they're balanced in a way where they just kinda end up as a poor man's SoA.
As it stands, they're supposed to be your group's 'sniper', or something like that. The dude that spots and weakens priority targets, but with more emphasis on blasting than say, a corr or defender.
Problem is that a soldier of arachnos, for all intents and purposes, fills this role 5000% better. Between their leadership, debuffs and general usefulness at every level with their leadership, they're so much better at the hybrid blaster/support role as a baseline, even with most of their mace/pack abilities being garbage, the sheer amount of utility offered by venom nade and their leadership buffs can't be understated.
Best way to look at sentinels would be to loosen some of the AoE restrictions in some cases and make the inherent much less terrible to activate. The way nuke damage calculation works on targets that aren't taped to your forehead is so unnecessarily restrictive.
And unlike other things with the game, they can't blame Paragon for this one, Sentinels are a purely HC creation.