Master Boy 2 (Gaelco, 91)
Why it's not emulated (Manuel Abadia):
No PCB for Master Boy 2 has been found.
Glass (Gaelco, 93)
Not playable due to protection, but enabled in MAME anyway.
TH Strikes Back (Gaelco, 94)

Pictures taken from
Emulatronia.
Sequel to Thunder Hoop, you control a
powerful ESP Rambo-like character, trowing sonic rays from his hands and
destroying mutated enemies. These are some WIP pictures, but because of
protection the game is unplayable.
World Rally 2: Twin Racing (Gaelco, 95)
World Rally 2 works okay, but if you crash the game goes a bit haywire
and doesn't work properly due to unemulated protection.
Info from Guru:
Again, [the emulation problems are] all due to the missing 32k of
Dallas DS5002 code.
Touch and Go (Gaelco, 95)

Pictures taken from
Tim's Arcade Page.
There are problems playing Touch and Go because of unemulated protection.
Snow Board Championship (Gaelco, 96)

Pictures taken from
Tim's Arcade Page.
Why it's not emulated (Brian Troha)
Unemulated Protection - test driver you can make some snaps from, but
skier is uncontrollable.
Target Hits (Gaelco, 94)
Some of the Gaelco games are unprotected and playable (for example
Alligator Hunt) but others fail the coprocessor test and are impossible
to control, among other problems.
Thunder Hoop III (Gaelco)
Description by Deb:
Similar to Surprise Attack, but without that strange
gravity... Player is manga-like, and enemies are genetic-modified women.