Double isn't very amazing on point. She's Doom tier in that she's got fantastic assists and DHCs that add utility to nearly any team-- LK Bomber is a semi-invincible lockdown, MK Bomber is a semi-invincible lockdown + zoning tool, catheads is a safe DHC, nighmare legion is a huge damage DHC, and so on. Her point options are limited but strong-- she has great hitboxes but mostly gets low/throw mixups and flesh step crossups that require you to already be in to be effective.
Filia's neutral is weak due to a combination of weak hitboxes, unsafe moves, the worst zoning in the game, linear approaches, terrible range, and an overreliance on meter for basic options (throwing, mostly). Once she gets in can do lots of mixups (provided she has the meter to fuel them), but this is no different from any other character in the game. SDE Filia was fundamentally different (notably, Launch Filia was considered terrible) because of unlimited airball cancels and ToDs for everybody off of basically everything. Now that airball and Fenrir have had their damage reduced to a fraction of what they were, Filia has the unique property of being the one character in the game who has to reset to do damage, rather than simply being given the option.
A great deal of Filiaphobia is due to people singling out Updo assist despite it being the worst of its kind, players being kept honest by Gregor (which has only a token pre-active invuln, no hitstop, and an abysmal hitbox that loses to nearly any other move), and the fact that they just don't play the character and thus don't know how to keep her in the helpless zone which is everywhere but point blank range. It's the polar opposite of the Complainwheel phenomenon (where an already fantastic character gets continually buffed because her players haven't stopped bitching since day 1).