By the time the DM is throwing disintegrate spells at your party, your Druid is reasonably likely to have more hp in their humanoid form than any of their animal forms, therefore if you rule against the Druid, they are in fact far more vulnerable than other party members when in beast form.
If you rule in favour of the Druid, you present them and the party with a disintegrate 'shield' of sorts, especially if the Druid tries to turn into a hawk, ready it's action and fly in the way of an oncoming spell. Granted, there're spells for that, but this method is always prepared.
On one hand, sometimes I look at the 5e Druid and go "Eh, boring", so might find their "Phoenix ability idea vs disintegrate" kind of cool. But at the same time, there are plenty of powerful raw damage spells that a Druid can do that with anyway. It's not the damage of the disintegrate spell that makes it special and iconic either, but the ash/dust effect, which is specifically one of the best ways to combat a creature with regeneration (I know this is different to Wild Shape, but the similarities are there)
On the other hand, I'm kind of a RAW person (bear with me here). I interpret that it's not the reduction to 0 hp that kills the Druid in beast form, but rather, that tiny fragment of time that the Druids beast form's internal organs including neurons in their brain turn to dust, however, as per RAW, I would say that as parts of the beast form disintegrate, just as instantaneously would the empty space begin to fill up with the flesh , clothing and objects of their humanoid form. Mundane objects would burn up mostly, but the disintegrate, once it eats up the body of the beast, would leave the druid's humanoid corpse, stripped of items, but intact.
This is probably the best compromise. Neither is gimped. Disintegrate gets it's pile of dust and dead Druid and the Druid got to change back into humanoid form and can be revived by a3rd level spell or a 5th level spell rather than a 9th level spell, which requires a 17th level healer.
I'd say that's a win win with a fairly RAW interpretation of the rules.