Well, a used automotive type of battery is likely to be better than a new ebike battery of the same cost, primarily because the EV-grade cells are probably higher quality to start with, and more likely to have been matched in characteristics (nothing to do with matching voltages) during pack manufacture, so more likely to stay balanced over most of their lifespan and give you more capacity for more of that lifespan. A cheap ebike battery (well, really most even the costly ones) is not likely to have matched cells, so is unlikely to deliver full rated capacity for long, if ever, and less likely to behave well under higher loads (vs the specs it claims).
Both are just as "closed system" (you can cut them open to fix them if you have to, but not generally designed to be repaired). So this is not any different with any battery, generally.
If you want to monitor individual cells, you could open up the case of any battery and wire in a battery monitor, either directly or by just wiring in a sense connector to the cells to connect external monitors to. Or add a BMS with cell-monitoring you have access to the data of (this could mean replacing the BMS or just adding it in parallel to the cell connections, and not using it's power connections for your system, only using the original one's). There are threads around the forum with more details on doing this kind of thing.
Regarding the 24v batteries: To use those in a 48v system, you'd need to series them. If the BMS in them is not designed for the higher voltage, then as soon as one of them shuts off it's output to protect it's cells for any reason, it will have full system voltage across the FETs, and they can fail. This usually causes them to stick on, which means the BMS is now unable to protect the cells--the output cannot be turned off anymore. So the cells can be overdischarged, or currents higher than they can handle be drawn from them, etc. This is a silent failure, so you won't even know it has happened most of the time, until cell damage has already occured (which can lead to fires), and pack behavior becomes different enough to be noticed by changing the bike's behavior. Sometimes the FETs fail open, so all power just cuts off and can't be restored by recharging or resetting. There are external electronics (diodes, etc) that can be used to minimize this problem, but it's more stuff (and money and effort) to add to the system and more stuff to potentially fail. There are threads around the forum with more details on doing this kind of thing.
