Proposal: Change currently useless units for balance reasons.
Reason: Certain units make very little sense to use. The game would be better balanced by giving them more realistic purpose.
Okay, then lets take a look at what you would like to change.
Details:
Chariot: spell summoning is useful, but otherwise chariots are a waste of research points when you can use hoplites
Cyclops
Minotaurs
Erinyes: slingers have a better attack/farm ratio and don't require favor
Centaur
Fire Ships/Trireme: these are useful for attack timing tricks, but that's it
Hydras: they have both offense + defense as compared to LS/biremes which only have one, but the favor requirements and god restrictions (can't support generic cities) make these basically useless.
That is no comprehensive chariot vs hoplite argument, yes hoplites are good for offense and defense, but they are silver guzzlers, and more expensive offensive units per farm space than slingers and horsemen, not to mention that chariots consume stone as their main resource, which is very useful for land defense cities that consume a tonne of wood and silver. Their defensive ratios are the between hops and chariots, 37 per farm space, but the flipside is that chariots defend slightly better against blunt (19 per FS as opposed to 18 for hops), and have more ranged coverage (14 vs 7), their only lacking is sharp, which tends to be the least common offensive unit barring manticores and medusas.
Cyclops are sub-par attackers, but they are decent defenders when packed with a lot of archers, the downside is the unit costs for both favor and resources, and a dangerous lack of sharp coverage. I have already made a thread on changing this unit, it is titled 'Rebalanced Cyclops, and includes lower costs and a higher defense ratios.
Minotaurs are fine, barring an arguable tweak, they are 51.1666667 defense per FS, and they specialize in blunt defense, like a divine unit equivalent of a chariot, but with 14-15 more defense per FS. They go great with swords and archers, making hoplites void if you can spare the favor costs, which aren't that high compared to many other myth units.
Agreed that Erinyes needs to be changed, I would say it should be re-made into a fast, flying, 25-27 per FS ranged attacker (1375-1485 attack), with reduced defense. This is an idea I have been mulling over for a while now.
Centaurs are fine as they are, 55 defense per farm space and the cheapest myth unit (100 favor), their defense ratio making them the equivalent of a divine archer, with a gap of 12 points to their regular unit counter-part.
Fire ships are fine as they are. They take patience and skill to use, which is the only conceivably balanced way to use a suicide naval unit.
Triremes, I agree, need to changed somehow, it is probably worthwhile increasing their offensive and defensive stats, and making them an effective breakthrough unit.
Hydras have the attack rate of LS while bypassing fireships, in a way that is a strong unit, yes it costs a lot to build, but I could only see some tweaking for this unit to make it more 'useful.'