Firstly, if you graft a Battlemaster with a slightly-promoted Adept, you get a free Govannon-style trainer, able to teach raise skeleton etc to anyone who happens along. If this isn't intended, the easy solution would be to separate out the trainer promotions
Secondly, if you graft a unit whose strength has already been improved by a weapons promotion, the new golem gets another bonus on top. This one's also exploitable by any other civilisation with mercenaries and mithril.
Using two OO high priests and a mithril-wielding mercenary, you can summon a kraken and graft it to the merc to get a free 14 str, empower x5 hidden nationality golem every turn. The mercenary is even kind enough to summon the next graft victim himself for a minimal fee.