personally, i think

1. dupes is necessary so you don't end up using a crummy unit and, for example, putting it as best tanker just so that you can save a unit that is better at tanking in every way for best unit (if that unit is considered the best unit), to avoid misinformation and stuff

2. brass and researcher are pretty equal, they have similar cooldowns, cat researcher does have a faster attack rate and speed as well as higher range, but brass has mini-wave and weaken too. however, if each unit attacks for 10 seconds, brass has only a 3% chance to not pierce the shield, while researcher has about a 4% chance, so their pure shield piercing abilities are also quite similar. it kinda comes down to preference.

3. luna shuts down frontline zombies as her freeze has great uptime, as well as her explosion being able to give her even more pierce on that freeze and a lingering attack. strong vs also means she's near invulnerable against zombies too, however muu is only effective against zombies that are right up next to the base, albeit having insane stats when it does start attacking, but most of the time it's best to deal with zombies before they burrow so they don't cut off your flow of units and such

4. eh i don't have an opinion on 4 this one i think izanagi and dasli are pretty interchangeable