Siege seems fine to me. You do have to worry about multiples in play and how to resolve combos with other cost-changers, but I've played quite a bit with
this card and enjoy it just fine. The stacking problem is certainly an issue, and I don't think I'd have wanted to publish it if I were Donald, but as a fan card for my gaming group, it works just fine.
I missed the first version of Great Warrior you had up, but the current version ("+1 card. Each other player discards an action card from their hand (or reveals a hand with no action cards).") still has problems. First, it doesn't help
you at all. The +1 Card just replaces itself in your hand, and now you're down an action. Having +1 Card is better than not having it, but nevertheless the full game value of the card rests entirely on the attack portion, which is only the case with the official cards in two special situations.
But the main problem is that the existence of an attack like this promotes boring money play. An attack like this is pretty significant, as most of your key cards are going to be actions, and so to avoid getting burned by it, you're simply going to forego actions and buy money. So games using this card are liable to degenerate into simple Big Money races
even if no one buys the card. Because the moment someone starts building up an interesting action-heavy engine, his opponents are going to pick up one or two of these to counter. And certainly they won't buy these
before anybody tries to build an action strategy, because then they'll be dead cards.
In a nutshell, even though the card itself is interesting, games using it will tend not to be. This is probably why there are official cards that punish money strategies (Cutpurse, e.g., and maybe Tribute) but none, really, that specifically target action strategies.