Here are some problems I've had with Marverni:
Ambibate Nobles are good warriors. However, they are not sacred, so if you build a ton of them, they will eat up a lot of maintenance. A combo I tried with Ambibate Nobles was to get Legions of Steel. That worked nicely. Except that once you get Legions of Steel, your casters will spam it on every unit you've got, even if it's a single Horn Blower sitting behind the ranks. Ambibate Nobles are expensive in terms of resources too. No way can you take any sloth.
The old age on Druids is a bit of a pain. Since Marverni has no blood/fire, it's difficult to stop them from aging. And they are
expensive.
Researchers are weak/expensive. Really you've got to take at least Magic 1 to keep up in the research game. Since you have no Air/Death/Fire, you can't build ANY research items unless you get lucky with an indy.
You have no bows. Sure you can mass slingers or javelins, but then you are even more screwed with upkeep.
You have no national summons. (Devs, how about sending at least some Cu Sith love Marveni's way?)
Nature: It's quick to get a Gutater to N3 but you need Const 6 or empowerment to get him to N4 so you can make Ivy Kings.
A Lamia Queen would be much better but you've got no Death. Even if you could make 1 Lamia Queen with a Pretender, it can't make its own Lamia Queens. Firbolgs are great thugs, especially if you have a druid casting healing light.
Earth: Troll Kings are kind of cool. But you have to summon the whole court, and that's expensive.
Astral: Golems are cool. But they're way up in Const 7. Sorry, no Ether Lords, because you guessed it, no Death. Perhaps an Astal/Death pretender to let you conjure Ether Lords might be a good plan, since Marverni does have a nice Astral base to start. Ether Lords could then make more Ether Lords. But at 90 pearls a pop, I think you'd run out pretty soon.
You have no killer rituals. I suppose Baleful Star is not bad, way over in Alt 5. But Air gets Hurricane in Evo 4 (Evo 4 is killer of course), Blood gets Toads in Blood 3 (duh), and Death gets Burden in Thau 5. Fire gets Raging Hearts at Thau 5 too. If Marveni had Death, it would be cool to cast Blight, but, hey, no Death. Melancholia is a good spell -- for someone to cast on Marverni.
Anyway, I still play Marverni, even in MP, because I like the theme. I'd lose in any case because of my vast noobishness
.
Marverni also does just fine in SP, since it's a lot easier to put up another castle and pump out a bunch of stargazers to support your druids, and you probably don't have to worry about getting rushed on turn 9.