Q:
Is this a correct assumption?
A:
If you are asking if the dropped exotics are equal to the dungeon and karma versions, the answer is yes. All exotics of the same level will have the same base stats (defense on armor, damage on weapons) the bonus stats (e.g. power vitality toughness) will differ depending on the prefix of course. If you are looking for a specific type of stat combination you may not be able to get it from drops. There are some types of stats that you get by crafting and some you get from dungeons or karma and some you get from drops. There is some overlap but there are some places you can’t get certain stats. For example you can get power/vit/toughness from dungeons or karma, but you can’t craft it.
Good luck, I hope you can get what you’re looking for. It will take you a while, exotics drop very rarely.
Crests (and their lesser variants “Medallions” and “Seals”) are equipment upgrades.
You can put them on armor instead of runes.
You can put them on weapons instead of sigils.
You can put them on accessories instead of jewels/gemstones.
They do not “overwrite” the inherent bonuses of the armor/weapon/accessory itself.
Statwise, Crests are better than gemstones, but inferior to jewels. However, some stat combinations (such as Crest of the Soldier) are not available on gemstones or jewels.
Compared to Runes, Crests provide a bonus that is “spread out” more (over three stats), while runes provide a bigger boost to a focused stat (or two).