I've been using D&D beyond character builder for a couple of years now and I'm finding it's becoming harder to use. The big issues is there's no support for things straight out of the DMG and no readily usable way of implementing them yourself. The biggest pains in the bum for me are:

 - There's no way to record XP past 20th level that I've found.

 - Adding spell slots. You just can't. You can add something hacky like a feat that gives a free action that says "use a spell of a given slot" but that means players have to go digging elsewhere to find out if they can use their spells.

 - Adding spells known. Short of copying and hacking the character's subclass to add spells there doesn't seem to be any way of doing this.

 - No way of choosing from a list of spells the character already knows. You might be able to do this with a subclass hack but I haven't checked.

 - No way of modifying the characteristics of a spell short of copying and hacking the spell. I really don't want to have an exponentially increasing number of spells which are all the same thing.

 - The only way to give a character an artifact is to copy and hack the item to include the random/DM selected minor/major boons/banes from the item. Also the "Book of Exalted Deeds" has to be hacked some more to get it to work as described in the DMG (you attune it, study it for 80 hours, then lose the item but get all the benefits from it).

Anyway, it's got to the point now where I'm stopping using D&D beyond for character management. It's more trouble than it's worth. Ideally what I'd like to see to come back would be all the Epic Boons and Blessings in the DMG implemented properly but I'll take enough functionality to implement them myself as feats.

 PS: Forgot not being able to set up a system of world specific languages. That's a real pain in the bottom.

For recording XP past level 20, I just use the Character Sheet's Notes section. I cannot think of a better way to track it, but at least it does not require a physical pencil.

For adding spell slots, I would just copy the Lucky feat and rename it "Extra X Level Spell Slots". It is the best temporary solution I can think of for now. Your players will just have to remember that they have extra spell slots on their feats page.

For boons and banes, I would homebrew each condition as a feat, and just have the player make a note from which source the boon/bane/"feat" comes from.

Yup, I found out the hard way about some of these like languages.

DDB works OK to do basic, standard stuff from the main rules. Try to do anything homebrew-y or optional and the whole thing falls apart. Which is why I really don't use it for much anymore, other than as a digital sourcebook. I do all the really important work in Foundry.