Note that only the main datasheet for a part cannot be enough.
Most (digital) parts have errata sheets, that require careful reading if you don't want to get stuck for DAYS at an odd problem. And you do want to have the most recent version, claiming (hopefully) all the bugs for each silicon revision.
Some CPU datasheets have separate user manuals, summary datasheets, or microchip even list each peripheral in it's own document. For a PIC32 there are like 24+ PDF's on 1 chip's page , all with useful information.
Then there are application notes, sample schematics/projects and/or evaluation boards that can be useful for designing as well. These are neither found on these generic websites.
Chances are you won't find these very useful documents easily (or at all) on alldatasheet.com or a like.
@Simon: User manuals or datasheets from NXP/ST ARM microcontroller parts can be a rollercoaster-ride-with-the-scrolling-wheel as well..