Today, tried to combine a 2014 species with a 2024 class. Did not work out too well. The species/class were a tabaxi monk.
The builder added in the +2 Dexterity from the rules for tabaxi using the rules from Volo's Guide to Monsters, rather than using the updated rules from Mordenkainen's Guide to Monsters. I wasn't given a choice in the builder. It also gave me the +1 to Charisma from Volo's.
Later on, the builder asked me where to assign my +2/+1 from the background... which it shouldn't have done after giving me the increases from Volo's. In fact, according to the 2024 rules, it shouldn't have given me the bonuses from Volo's or Mordenkainen's but only the adjustment from the background.
In any event, I ended up with my Dexterity too high, and my Charisma too high and I couldn't find any way to adjust them down.
Further along, when it came time to fill out the "About" screen, I was asked to choose only one language other than Common. Again, this is the rule from Volo's, not the 2024 rules. According to the 2024 rules, I should have been given the choice of two languages in addition to Common.
But, the system failed with regard to movement. I was given only the base walking speed of 30. I wasn't given the climbing speed according to Volo's (20), or the climbing speed according to Mordenkainen's or the 2024 rules, both of which would have been 30 - the same as my walking speed.
I ended up with the wrong AC. It gave me the Unarmored Defense calculation of 10 + Dex + Wisdom but, since my Dex was given an additional +2, the AC calculation was wrong and I ended up with an AC that was 1 more than it should have been. Because of the additional +2 to Dex, my Dex-based skills are all wrong too. And Initiative. And Dex save.
I was given proficiency with the Hand Crossbow which is incorrect. The 2024 rules give Monks proficiency only with melee weapons. Monks do not get any ranged weapon proficiencies. This is wrong in the online compendium too. To be fair, a number of websites have this wrong as well. But, the printed book and danddbeyond both say "melee weapons". I've reported this before. Several times. It doesn't seem to get fixed.
For some reason, Roll20 has chosen to list "Martial Arts Attack" and "Unarmed Attack" on the sheet separately. The "Unarmed Attack" listed is the base attack that every character gets. It shouldn't be there on a Monk sheet. And, the attack for all the monk weapons should use the Martial Arts die as well. There doesn't seem to be any way to change the damage for these attacks. This would leave me having to delete all of the attacks that had been added to the sheet, including all of the "ranged" attack lines for the dagger and spear, leaving only the "Martial Arts attack" line. The dagger and the spear are not ranged weapons so the Martial Arts die applies even when thrown.
There's a line on the sheet near the bottom that says: "Martial Arts" with an on/off slider next to it. Moving the slider to the "on" position changes the formatting of the sheet slightly. It also changes the AC. I'm not sure why because the "Martial Arts" feature doesn't affect the AC. And the sheet gives a +0 mod for the Martial Arts and the "Affects" column is blank so someone knows that it doesn't affect the AC but it changes the AC on the sheet anyway. Now, if it was for "Unarmed Defense" it would make sense but that feature doesn't appear on the sheet with an on/off slider.
There are probably more errors on the sheet but finding them is difficult with all the already listed errors. The TL;DR version is: the sheet for the 2024 Monk class with a 2014 species is a total mess.