I had to remove steel plugs from an aluminum head on my '92 Dodge I.R.O.C. RT Daytona and it was a real ballbuster. I think that partly it was the different metals sharing different expansion rates and a corrosion between them.
I also gave up trying to get a long steel bolt out of an aluminum alternator on my Fiero. Wound up using a carbide cutter to wack off the bolt head and nut and removing alternator. Once off..it was as though the steel bolt and aluminum alternator chemically welded themselves together!
I used "never-seize" compound on the new bolt and replacement alternator on the Fiero hoping it'll come apart if need be the next time!