I think it is hard to understand for people who have never had to handle multi-threaded coding or something similar.
If you allow multiple players to act at the same time you have to decide how you isolate them.
1) Do you make it so that no one can see what the others are doing until the phase is over ?
This means that the server must retain information from the players, that's extra work to define exactly what is retained and form which players.
It also means that players have to wait longer until they can see what the others have done.
(This is what 7 wonders does)
2) Or maybe you can see what the others have done once you have submitted your turn even if the game still waits for other players ?
Here too it means that the server must retain information from the players.
And you cannot allow players to cancel after they have completed their turn even if the table is still waiting for other players because that would give extra information just to the players that do a cancel.
(This is what railroad ink does)
3) Do you allow players to see what the others are doing as they are doing it ?
This can be problematic in some scenarios where there is an advantage to wait for other players to decide...
And you might get interruptions if other players are playing at the same time as you (like in It's a Wonderful World)
Here you have to make sure that pushing the information related to one player is not disrupting the other players clients because those clients now have to be able to receive information about other players while the player is acting on the client. And it has to work even if the player does a local cancel of their actions.
4) Other...
Some games like Puerto-Rico allow the player to force a return to sequential play.
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IMO, systems 1 and 2 are much more comfortable as a user. (1 or 2 is best depending on the game)
No matter what the global approach is, there might be some specific elements in the game that compound the difficulty of implementation.
Take for instance Race for the Galaxy, it mostly goes smoothly with simultaneous turns (mode 1) but many powers (like draw and discard effect in phase II) are handled via extra steps that must be done before anyone else can do the main part of the phase and the rare PvP conquests are bugged bugged of this and it hasn't been fixed in ages because it's delicate (maybe it's fixed now, I didn't check recently but my point remain). You want phase 3 to be simultaneous but those rare PvP conquests cannot be and the military power of someone is impacted by what they play in phase 3 so those impacts should be "put aside" until the end of the phase so that those fights can proceed as according to the rule. And still for RFTG, for a long while, there were also problems with objectives because hovering on them could give you information about what other players are doing during their simultaneous turn which is very relevant in that game.
It is not rare for a real life game with simultaneous turns to have special rules or FAQ entries about what to do in specific scenarios where one suddenly really wants to know what the others are doing before doing their own turn and vice-versa and or how some specific cards or actions are to be handled in such situations...
To go back to Caverna, there are many things that you can do during harvest depending on the tiles, gems and resources you have. And some of those things will inform other players as to your intentions for the next round. So deciding what is visible when, is relevant.
