Much of the grumpiness you might see regarding the Codex falls under two categories: bugs (of which there are several, many of which have been in the Codex since it was launched), and disillusionment due to unmet expectations.
Many people, on hearing that a "Codex" was being introduced and it would be intended primarily for explorers and would list the finds and discoveries that explorers make, were hoping it would generate a list of
all of your discoveries, so that, if you've discovered 129 Earth-like planets, it would give you a list of all 129 of them. Not so, apparently, and it was never the intent or design of the Codex to have such functionality. But since this was never actually stated in the lead-up hype, and since the early demonstrations did seem to imply that such a list would be compiled, many people were kind of upset when it didn't.
I, personally, consider the Codex as just one more piece of evidence that FDev really, really don't understand the Explorer mindset. Other examples include:
- That early Q&A video session with the Devs, where someone asked if they would be able to fly all the way to the Galactic Core. The Devs looked at each other in puzzlement and said "Sure, but it would take a long time, and why would anybody want to do that anyway?".
- When the CQC update landed, it caused a game-breaking bug for explorers (old scans were lost, new scans wouldn't save properly). To their credit, FDev worked overtime and fixed the bug in a week or two, but while they were waiting, their instructions were "Just stay where you are and don't jump to another system, or scan anything". So, explorers trapped out in the black were asked to simply not bother logging onto the game for a couple of weeks. "Oh, but while you're waiting, why not try out CQC? It's great!". Yeah, nah. Most explorers hate combat, they're not going to fire up ED just to play a combat mini-game.
- When jet-cone boosting was added to the game, neutron stars were only supposed to provide a 25% range boost; due to a typo in the coding, they were accidentally given a 400% range boost. It took much begging and pleading from explorers for FD to not "fix" this "bug". I seriously doubt anyone would have ever risked their ship in a neutron star cone, just for a 25% boost.
- Fleet Carriers. Nuff said there.