Statuepode with a shield has a better damage dealt/damage received ratio than a dragonpode in almost any fight
This is not true, especially if you change from "damage dealt/damage received ratio" to "current hp/maxhp at the end of the fight", since the latter is what matters. (You can use the former as a proxy for the latter in all cases outside of forms, but dragon form gives you more maxhp than statue form, so it fails here.)
It is true against things that just do damage by meleeing you (I think? I'm too lazy to really test this appropriately), though not actually by a lot, and it sometimes depends on you actually having a shield.
Against things that have unblockable conjurations (including stuff like damnation) the shield does nothing and the AC from statue form tends to do less (conjurations deal a lot of damage) so dragon form's higher maxhp and higher damage is a big deal.
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If you are using a shield, then you are purposely trying to make statue form good and (maybe you don't even realise this) things like dragon form bad (dragon form can't use a shield, so it is just wasted xp that could have gone somewhere better). Obviously, if you go out of your way to make statue form better than dragon form, statue form will be better than dragon form. This might be the right thing for you personally to do as a player, depending on what you are good at. It's very much not the case for me.
It might be illustrative to think about blade hands compared to ice form, also. There's no slowness to complicate things, so this is simpler than statue form-vs-dragon form. Also, ice form and blade hands do basically indistinguishable damage from each other in most situations if you aren't worshipping chei. So the only difference is in the defenses you have in either form. Blade hands typically gives you better AC and better EV; ice form gives you better HP and no spellcasting penalty. From playing characters built toward using one or the other of these forms, I personally think that both forms have advantages (and I think I do better with ice form than with blade hands).
Almost everyone builds toward making blade hands good instead of making ice form good, so then, yeah, no surprise, blade hands does better than ice form when you have a +7 swampDA and lots of dodging skill and less fighting skill and you didn't even learn any escape spells you would cast mid-fight. The mistake is then concluding that ice form is bad without also trying to build an ice form character. If instead you try to make an ice form tm (wear robe, train less dodging skill, no armour skill, more fighting skill, maybe you even have some escape spells) then you might find that ice form actually outperforms blade hands on that character. It certainly is true for me. You might still find that blade hands outperforms ice form even after trying this; that's completely believable (lots of people find ogres difficult, and ice form is a lot like ogres).
Just as you can't build a blade hands character and then use just that to conclude that ice form is bad, you can't build a statue form character (like octopode with shield and lots of dodging skill and not as much fighting skill) and use just that to conclude that dragon form is bad. Maybe players legitimately don't recognize that you can do this and it matters?
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If you want to fight a big group of enemies that is just going to hit you with melee attacks, statue form is pretty good. Personally, however, I find situations like that very low-threat to begin with, since my personal strengths include threat evaluation and melee tactics to fight things one-on-one. So using ice form or hydra form or dragon form in those situations, while it is a bit worse, is still perfectly fine for me; I don't die either way. Scary spellcasters are better tackled with dragon form, since the best defense there is just killing them as fast as possible. Those are the enemies I tend to die to (outside of moth-of-wrath-related things in zot), so I find dragon form better than statue form in general. But, of course, this isn't just a "you can only pick one" sort of thing anyway.
Other players do die to things just meleeing them to death, and if this includes you, then you will surely find statue form better than I do. Statue form is pretty much unarguably good in tomb. Dragon form would be unarguably better against orbs of fire and similar enemies even without the rF it provides.
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note: I have been very careful throughout this post in trying to keep things that are from my perspective clear. I do not claim that what I am good at is what you are good at. Maybe you are better at getting out of messes than I am (I'm really bad at it) and thus you can handle when things start to go wrong in statue form better than I can. Maybe you're worse at threat evaluation so you need the buffer that statue form gives you. Those are things that would make statue form relatively better. I think most people find blade hands superior to ice form even after trying an ice-form-built transmuter.
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Hydraform is fun but all offense, no defense; it's a good form if you lack statue or dragon, but rarely useful after either.
Hydra form is pretty much exclusively useful when it is vamp-axe form. If you're attacking things it eats during fights then it is really very good. If you're not, then it's a bigger ice form but smaller than the biggest ice form (dragon form) so yeah, not very useful. However hydra form is actually still useful for a statue-form character, since it's different instead of just worse.