do threat missions replenish? it says nothing about them replenishing, but do they?

Once a threat mission is played, it is returned to the box. New threat missions are not automatically added.

Mission results may explicitly make a new threat mission active.

Has anyone had a really tough time in Hoth as rebels? some of the missions seem imposible. The rebels won the first two story missions and one side mission and then lost the next 4.

Hoth is supposed to have a Rebel slant, I believe.

That being said, there are some missions (notably Return to Echo Base) that make things more difficult for the Rebels.

I suppose if the wrong Threat Missions go unchecked, it could make things go pretty badly for the Rebels. For instance, Supply Disruption probably isn't going to ruin any mission for the Rebels (though its effects could possibly be felt on the course of a campaign, if the Rebels like getting crates), and YMMV with the extra influence from Constant Advantage, but Distraction or Weary from Toil can really shut the Rebels down (especially when paired together).

Right now I'm in the middle of a Hoth campaign (Mak, Fenn, Murne, and Vinto v Armored Onslaught), and we've played:

The Battle of Hoth (Rebel Win, decisive but not a total blowout)

Sympathy for the Rebellion (Rebel Win, total blowout)

White Noise (Rebel Win, down to the last terminal, but I would have never gotten it)

A Light in the Darkness (Rebel Win, but very close)

Forest Ambush (Imperial Win, huge upset and total blowout)

Return to Echo Base (Imperial Win, very close- down to the last action, probably)

Rescue Ops (Will play next week)

I not have Supply Disruption, Weary from Toil, and Distractions as Banes. If the Rebels don't start attempting Threat missions, I think that might really come to bite them in the butt soon.

The Rebels had a really strong early campaign, but things started changing after White Noise when I picked up Mortar. It's a strong card, and while I don't think it's as totally gamebreaking as some people claim (at least, not after the first turn) my Rebels haven't quite learned how to deal with it yet and it's been really tough on them. We play our next mission next week, but I've noticed that the player who controls Vinto and Murne has been saving up quite a bit of XP, so it'll be interesting to see whether or not the new high tier abilities that they gain sway the campaign back into the Rebels' favor.

1 hour ago, Corver said:

Has anyone had a really tough time in Hoth as rebels? some of the missions seem imposible. The rebels won the first two story missions and one side mission and then lost the next 4.

Dunno, as the Imperial player I felt like the rebels knew what I planed to do like 2 or 3 rounds ahead ... I might also add that I played the whole campaign solo

More seriously, I have also read that RtH is supposed to favor the rebels. Although the poll on BGG says that some missions seems hard to win has rebels (like the return to echo base interlude mission). The poll is great, however it does not tell how close (or not) went on each mission. Although 80% of the voters acknowledged an Imperial victory in the interlude mission mentioned above, if the games were close games on average I'd be fine with it. I guess I'll find out as I'm starting the campaign over (as the imperial) against 2 friends (real friends that others can see ) this Friday. Can't wait!

42 minutes ago, subtrendy2 said:

We play our next mission next week, but I've noticed that the player who controls Vinto and Murne has been saving up quite a bit of XP, so it'll be interesting to see whether or not the new high tier abilities that they gain sway the campaign back into the Rebels' favor.

So many factors can affect "balance" of a mission and you pinpointed one right there. Two heroes holding back on spending their xp at the same time can have a major impact against the rebels up until they acquire the 4xp cards.

1 hour ago, IanSolo_FFG said:

So many factors can affect "balance" of a mission and you pinpointed one right there. Two heroes holding back on spending their xp at the same time can have a major impact against the rebels up until they acquire the 4xp cards.

Oh, absolutely. Balance in Imperial Assault is more of a reference to the fact that the game has potential to be balanced, not that every mission has an equal opportunity for both sides.

Anyway, I think OP's question has been answered.