My objection to them locking us out of start room is that the current design of the 5/5 encounter is unbalanced from every other boss fight in Dragon Age's history. Except maybe some of the totally unreasonably stuff in DA2, but who really wants to accept DA2 as legit game/level/encounter design?
Assuming all the bugs (like armor not actually applying its armor value, and errors in agro mechanics, ranged attacks clipping through walls and cover) get fixed, they need to change how enemies spawn in. Currently its just a spawn pattern up to the limit of AI on-screen. There is a defiant Unit Cap to the number of AI that can be active. As AI are killed new ones are spawned in to replace those killed.
This creates a never ending rolling wave of enemies that come from almost unmanageable directions. If this were an SP Boss Fight, it would be considered one of the most frustrating and worst in the game. BioWare would be decried for bad encounter design, as they should be for this
What fundamentally needs to change to make playing outside that starter room even viable is how the spawn waves work in 5/5. It needs to actually spawn in Waves, with breaks. With very clearly defined unit composition balance (between ranged and melee) in each wave of additional supporting enemies.
Because of the never ending wave that happens on all Factions, the fights end up playing out the same. 1) Kite or Tank the boss until the adds pool is depleted. 2) Burst the boss (Venatori), then deal with rolling add wave. The only variation is dealing with the individual boss mechanics, which isn't enough to be truly distinct.
5/5 is the most minimally designed encounter I think I've seen in a released video game in the past 20 years. Its design is Spawn Boss, spawn adds up to Unit-Onscreen-Limit, have all rush player. You can throw this together in Project Spark in about hour, give or take designing the Boss enemies mechanics.
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Until BioWare actually designs some boss fights for 5/5, I feel totally justified in using terrain advantage to mitigate a Mindless Horde Rush.