Raid Counters including Legendaries

i think it would be.

lugia has insanely high base defense. it would have been an incredible mon to throw in a gym but its definitely not a great choice to attack with. dps and attack stats are very important for raids. venusuar is weak to so many better options. psychic for ex: use those kazams/espeons/exxecutors with psychic moves. also weak to fire - flareon/charizard. not to mention its weak to flying- a dragon with hurricane - (another note- compared to legend raids where survival plays a much bigger role level4 raids are far more doable without dying- certainly not needing as many people in the group- dont be afraid to use a glass cannon- maybe throw a tank like lax as your 6th guy)

i understand that its fun in theory to powerup /use a legend but to me its a waste of dust that is better used for other mons. (unless you literally are drowning in dust and it doesnt matter- even so its just not a ideal option...so many better choices)

Also a very good counter into Machamp. Double resistance against his fighting moves and SE flying and psychic attacks.
As already mentioned, with severe time constraint, counters with higher ATK might be better nevertheless.

I think one should take into account the cost and time of powering up a Lugia for the purpose of a venasaur specialist.

The attack stat of Lugia is just too low, as mentioned multiple times already. In raids she might be useful as No. 6, a safety net for staying in the fight. That is, she would act as a better Blissey. :-)

Lugia should be good for sweeping normal gyms. Being very tanky and with attacks few defenders resist she could probably clear it by herself, saving you potions and revives.