I am not surprised that the conversion guidelines are weak, but I am always surprised about how many people eagerly awaits for them, as if they were so important, and are then inevitably disappointed, as if it was possible not to be. Most editions are just too different in both the mechanics, the concepts and the availability of things. At least in 5e character restrictions are at an all-time low, and the PHB has a high number of races and classes, so it's unlikely that someone has to drop too many defining features of their PC. But if your defining features were mechanical, then you shouldn't even expect to be able to recreate the same thing in another edition, with few exceptions.
That said, the conversion guidelines do provide a couple of giddy moments for sure:
- "Level in other* editions converts directly into fifth edition" (other than 4e)... really? How about those older editions where classes had different XP/level progressions, so that a Thief of level X was usually a lot weaker than an Elf (class) of level X? Classes were not equal at the same level, so they had different progressions for balance. If your BECMI party was fair with a 15-lv Thief and a 10-lv Fighter, your 5e party will not be fair with a 15-lv Rogue and 10-lv Fighter.
- "Choose the race that most closely matches your character’s race" and "Choose the fifth edition class that most closely matches your character’s class". So if you were a Fighter in previous editions, you should be a Fighter in 5e. Who would have thought? But be careful because "Class names can frame your thinking"! You never noticed that Magic-User, Mage and Wizard were really the same thing before these guidelines. <sarcasm off> Maybe what they had in mind here is rather converting PCs that used non-core classes and races. Except that maybe you should first think if it's really worth trying... why did you enjoyed playing that class or race in the first place? If it was because of the story, then surely go ahead, keep the flavor intact and just build a new PC of a class/race that exists in the 5e PHB. If it was more than just the flavor, you'd better play something else until such class/race becomes available in 5e.
I do like how they suggest to just ignore feats from previous editions (often about fiddly bits, sometimes individual special abilities) and use 5e feats in a rather different way. And I also like how they clarify that old editions really meant Lawful vs Chaotic to represent the heroes vs the villains, and that is very different from what they meant after the introduction of Good vs Evil axis (and yet a lot of people still think that a chaotic character must "sometimes" do something evil or plain stupid...).
Bottom line, count me in with those who don't see why bother with converting older-edition PCs with precision. Most people just all the time create new PCs, very few keep playing the same PC for many years, and if that's your case I think you'll have more satisfaction in keep playing such PC also in the same edition, rather than suffer the inconsistencies with a new one. Just try something new in 5e.
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About converting adventures and campaign settings, that is something a lot more interesting. You might want to still use those 100s of adventures you bought years ago and missed the chance to play when the relevant edition was current, and you might love a fantasy setting but not the rules of the edition it was written for.
Adventures tho are easy to convert. All you really need, is to decide (or figure out) the appropriate level range, and make sure that nothing (or at least not too many things) deviates too much from that. Maybe you had an adventure against Werewolves, and in that old edition Werevolves were fearsome foes not suitable for any party under level 10, while in the new edition they would be push-overs for such PCs. Well you can just use 5e werewolves, figure out how difficult the adventure is in 5e terms, and run it for appropriate PCs. If your PCs are already too strong, boost the werewolves using the DMG or just put more of them. Do something similar to all encounters so there is none which deviates too much from the level range. After you've set the encounters, all the other non-combat challenges (traps, various tasks and checks needed) should just be given a DC using the Basic/DMG guidelines.
Nothing needs to change in terms of narrative, unless you had unique monsters. That's the only difficult part, but there is already stuff in the DMG to create monsters from scratch. It might be just easier to use that instead of trying to adjust the monster and balance by addition and subtraction.
I think overall the proposed "Careful Conversions" are just good advice, and I would bet they give more reliable results with a small effort, compared with the "Quick Conversions" that seem actually more precise but less reliable.
Campaign Settings are a lot more difficult to convert, at least if you want to have mechanical character options. Obviously if you just want to keep the narrative, there is near-zero effort required. But often you want unique races, unique classes, elite options (e.g. older edition's feats, spells, prestige classes/kits/etc.) and possibly dedicated rules for stuff that isn't yet in 5e. The problem with that is that no official conversion document can ultimately do the work for you, only a full sourcebook can!
