The big thing about Shapechange, to take your example, is that it tops out at twice your caster level in hit dice. Druids are, I believe, limited to the Druid's level.
A Scroll of Shapechange would cost 9x17x25 gp, or 3825 a pop, and 153 Exp to create. Cheap at high levels, but expensive enough not to be used as a casual thing.
And, of course, the Fighter would need to roll a 37 on his UMD check to pull it off without mishap. Tough roll for someone who had to cross-class the skill.
(And not every DM will allow your artificer to craft a +20 UMD item. Plus the Artificer needs to actually have 20 ranks in the skill to make it, so ... )
Still it would be fun to watch it happen, if only to see what kind of creative spell mishaps the DM comes up with when the dice roll fails by more than 10.
Here's a fun trick, for someone intent an abuse: The feat Master: Staff lets a spell caster burn a spell of the appropriate level to power a staff, instead of using the staff's charges. There's a caveat in there regarding specialized casters but the wording is funny. As written, the caster can use a bonus spell from their specialty, but can't lose it, so by a strict reading of the RAW they could fire that staff forever.
The exact wording is...
Benefit When the character activates a staff, he or she can substitute a spell slot instead of using a
charge. The spell slot must be one the character has not used for the day, though the character may lose a prepared spell to emulate a wand
charge (the character may not lose prepared spells from his or her school of specialty, if any). The spell slot lost must be equal to or higher in level than the specific spell stored in the staff, including any level-increasing metamagic enhancements. A character cannot emulate a
charge for a staff function that does not match a specific spell.
I suspect that the word "wand" is a typo, a leftover from the Master: Wand feat.
So, as written, they can substitute any spell slot of the appropriate level, but can't "lose" a prepared spell from their school of specialization. Not "can't use" or "substitute", but "may not lose".
Of course, it requires a DM willing to let you rules-lawyer to a ridiculous degree.
