There is another thing I've discovered about merc units, which I'm finding slightly annoying at the moment. Situation is as follows:-

1) Merc unit is not a type your faction is able to train, so replenishment or blending into your own units is not possible anyway.
2) The unit suffers losses in battle but after a defeat, say, some are returned to you as prisoners. So the unit is now split into two halves.
3) You move the halves into a common province, planning to merge them the following year.
4) The game refuses to let you merge the two portions, even though they started life as a single unit.
5) Similar situation when there were two merc units of the same unit type which get depleted sufficiently to merge into a single unit. Again it won't allow them to merge.


In some ways, the behaviour as in item 5) is probably realistic, given that they'd be fighting for their own shares in ransom money and so forth, so one of the two generals in charge refuses to accept a merger.


Interestingly, with merging my own units, I sometimes find that it is allowed but 1 man gets kept separate, even when the bigger portion is still at less than its default strength. The common feature I've observed is that both of the generals concerned have developed V&V characteristics of some kind. It won't allow you to merge that oddball/reprobate (of whatever description) into non-existence within a better unit!

Then again, I guess if he is left behind as a 1-man unit, he can then be disbanded...