I've said this before, and doubtlessly i'll say it again. The first 150 is a very unbalanced group of Pokemon for 1 major reason:
No Effective counters to psychic types. There are no dark-types, the one ghost family is combination poison, and there are no effective bug-type attacks.
That combined with Mewtwo's existence in the first 150 (and bear in mind that Mewtwo is *still* considered top tier, some 500 additional Pokemon later), then add the fact that there is literally no effective anti-Mewtwo countermeasure in the first 150, and you end up in trouble.
Monster Hunter: pissing me off since 2010.