OP exaggerates the difficulty. You can trivially parallel the vast majority of lithium batteries so long as their voltage is reasonably close (I personally wouldn't fuss much over a 100mv difference, or even more in most cases, unless it's a massive battery or a power cell capable of delivering and accepting very high currents - charging most cells will often involve raising the voltage 200-300mv during the constant current phase, so you can safely parallel with a difference like that)

You can match up pretty different batteries in parallel as well. One will take more load etc, but this is not usually a problem. It's not ideal, but I think people often exaggerate the dangers.

Series is much more problematic, since most balancing circuits have very limited capacity to balance mismatched batteries.