Mixing modules

Key bit is voltage - if the chemistries are similar so the SoC vs V table is similar…

and only if paralleling to increase capacity, never try to stack serially to get higher voltage.

But yes sub optimal, more care, knowledge and gear needed to keep things working smoothly

Can you elaborate on why not to stack serially?

Because the smaller capacity cells will fully charge before the higher ones, then things get all 4th of July and explodey.

I get your meaning, but I would expect the BMS to stop before that happens, but then you'd have a bunch of cells undercharged. Okay good enough, no mixing for me.

They'll also discharge at a different rate than their cousins

For serial connections, each member of the string really should be as identical as possible in V vs SoC, capacity, wear level, ESIR.

The pack is only as good as its weakest link, and cell/groups in poor health will accelerate the aging of the pack as a whole.

Plus you must have one BMS control the whole string, not separate ones per group.

With paralleling only gross failure of one member impacts the whole. And each sub-pack can more easily have its own BMS if you want

simplest reason is that any capacity above the lowest can't be used and is dead weight. Once the lowest capacity cell is at its lowest acceptable voltage, the vehicle must stop, regardless of how much the others have left in them. I know of a company that rejects hundreds of thousands of dollars worth of cells just to get sets that are within a percent of each other. There is a very good reason for this.