LiPo/Li-Ion Cells in series max end-to-end voltage
There are multiple issues, the most important are personal safety, and the safety of the cells.

Most people think of batteries as 'safe', and may be less careful around them than with mains electricity. Different authorities across the globe have slightly different voltages for when low voltage stops being 'safe', but all pick a voltage in the range 40v to 60v.

Many applications manage this problem by building cells into 48v batteries, which are relatively safe to handle, service, replace, and then connecting those in series if they need higher voltage.

When you build cells into a battery, some battery chemistries like lead acid are sufficiently tolerant of cell voltage that you can infer cell voltage from battery voltage. Similarly NimH, but to a lesser extent. Lithium chemistries are totally intolerant of this, any lithium battery of any size needs individual cell monitoring. So given that you must monitor the voltage of all cells, the complexity of the monitoring circuit becomes a practical issue. It's relatively straightforward to buy or design monitoring that copes with 48v batteries.

It goes without saying that any cells connected to make a battery should be of the same type and manufacturer, state (ideally new) and state of charge.

Once you have a battery, you have the potential difference between the casings of cells to consider. This will rarely be a problem in normal operation, but the robustness of insulation may be the difference between an accident being inconvenient, and causing fire, explosion or loss of life. Another good reason to keep the end to end battery voltage lower rather than higher.