Two separate issues.
1> Is Adamantine too good, in that it allows you to bypass walls, doors, etc?
Personally, I wouldn't say so. Sure, it's a good thing to be able to bypass all of these, but by the time you've reached moderate levels, your Wizard is using Passwall, Teleport, Disintegrate, Fabricate, Dimension Door, and so on to bypass these anyway. Simple physical barriers just aren't anything more than a speed bump once you get to a decent level.
It reminds me of a scene from Piratecat's Story Hour, where the party (mid-teens level) had been imprisoned, but the guard left the door unlocked. When asked why, he responded, "Could you escape if you wanted to?" "Yes." "That's why."
2> Adamantine vs. Adamantium?
I just go with something based on a variant I saw in an AD&D supplement, and say that they're two distinct materials.
Adamantine is the "pure" metal, with big bonuses (keeping the 3E nonmagical Enhancement bonus) but it disintegrates in sunlight. It bypasses Hardness entirely.
Adamantium is a less powerful but more stable alloy that works anywhere. It still bypasses DR X/adamantine, but only ignores half Hardness (up to a maximum of -10). It's also cheaper and a bit easier to work with.