I do not think Detriment is inherently weak, as such. Controlled by a strong player and used alongside a ball of Lobsters I have seen it do some very powerful things. I think that Detriment is unpopular because it is
risky.
If you have the resources to build Detriment, you are most likely ahead in the game. If you are ahead, you want to stay ahead and make small incremental gains rather than doing anything risky. Bertha, Nuke, Paladin, and the superweapons are all by design much better at staying safe than Detriment.
[Spoiler]This is similar to the reason why I think Ravager and Hermit are not terribly good units. The stat numbers on Ravager and Hermit look appealing. But "low-hp-per-unit assault" and "late game big risk" are just not very appealing roles.
Running the argument in reverse, Badger on paper is really clunky and has really terrible stat numbers, but "can store up damage and wait until enemies get in range" is sometimes a really powerful role.
I'm not sure I like the idea of directly buffing Detriment, particularly by decreasing its cost. That might make Detriment more popular but it would also increase the number of games decided by a "does the other team manage to stun/ultimatum/etc the detriment" coinflip, which I do not think I would enjoy. Balance changes should aim to improve the experience of playing Zero-K, not just to fiddle with which units are popular and which are not.
Besides which I believe that Detriment does have a role in Zero-K outside team games. In team games it does not matter how fast you break the enemy defences and kill their base, as long as you succeed. In FFA games, on the other hand, moving quickly can be very important (read: worth accepting some risk), and Detriment will force a result much more quickly than Paladin while not painting a giant target on your back like a superweapon would.
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Djinn is already a good unit in any game where you can afford to spend 800 on logistics. Naturally this is not every game. I primarily use it:
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To move an army from one front to another quickly. This does require you to have realised in advance that moving that army will give you a strategic advantage.
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To rescue units which have dived into the enemy base.
Hacksaw and Quake exist to do specific things, they are pretty good at those things, and probably shouldn't be too good at other things. Besides which, is Hacksaw even bad in current patch?
Newton is weird but has several uses.