So I decided to run some numbers to see whether they are in fact better than hoplites... and to summarise - when they receive their 50% attack bonus they are about as good as hoplites on a cost / pop basis, and when they do not receive this bonus they are in fact inferior to hoplites!
Let us compare and contrast:
FU Zeus hoplite: 2 pop, 50f, 40g. 12.8 hack, 57% h.a. and 44% p.a. 172 hp
FU Myrmidon: 3 pop, 70f, 50g. 18 hack, 59% h.a. and 47% p.a. 165 hp ........ 50% attack bonus vs most non-greek human units.
I think that it's fair to "boil this down" to each units' stats per pop slot, and for simplicity let's discount the effect of armour as both units when FU have very similar armour:
1 pop slot worth of FU Zeus hoplite: 25f, 20g. 6.4 hack, 86 hp.
1 pop slot worth of FU myrmidon: 23.3f, 16.7g. 9 hack (including 50% attack bonus), 55 hp.
SO the myrm's 41% attack advantage over the hoplite is negated by the hop's 56% hp advantage over the myrm. Per pop slot the myrm is slightly cheaper, which I think evens things out... but the Zeus hops move faster which is useful.
So in situations where the myrm utilises its attack bonus it is comparable to the hoplite, but against greek opponents, TAs and Eggy axemen it receives no such bonus, whereas the hoplite remains as strong and therefore the superior choice.
Given the above, the only advantage that I think myrmidons might have would be with beast slayer vs myth units... but if you are playing as Zeus you'll have Bellerophon on the field and myth units wouldn't last long anyway.
So have I missed something here, or are myrmidons inferior to hoplites?