What are Pumbloom's and Missingno.'s base stats, out of curiosity? They're a part of my team no matter what (Pumbloom is adorable, and Missingno. is a classic - and it would be remiss not to use a pair of unique mons like them), so I'm not asking for /that/ sort of reason. Pumbloom's stats seem all-around low - perhaps to make up for its decent learnset? Missingno. seems the opposite - stellar stats (especially defense and special defense) but shallow movepool.
For anyone curious about the learnsets, these go up to lvl 55. Pumbloom seems to learn a move every 5 levels, and Missingno. every 10 levels.
Pumbloom (Grass/Poison) learnset
Missingno. (Water/Flying) learnset
Both of their TM learnsets are pretty shallow (Missingno.'s is certainly nowhere near as broad as the actual Missingno.'s TM set was in RB); the only really good compatibility that I've found is that Missingno. can learn Blizzard. Of the HMs, Pumbloom learns Flash, and Missingno. learns Fly, Surf, and Whirlpool (and maybe Waterfall, though I'm not sure if it's in the hack (have just defeated Mauro, so pre-Elite-4)).
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I really liked Missingno.'s place in the story, by the way (and since it's relevant to that: Flynn was a great fight!), though it would have been neat if some folks in Cinnabar mentioned them too (it's possible that Missingno. are the reason why the eastern shore is walled off, but that could just be the remaining lava flow from GSC's Cinnabar map, plus I didn't find any NPC who talked about it).