For my other DM's out there, how often do your PC's background abilities come up? Like Sailors getting free passage or outlanders finding enough food?

How do you handle some of the more useful ones? I made, what I thought was going to be a survival heavy game with hunting and gathering mechanics and everything. What ended up happening is 3 people picked outlander, survival proficiencies and the goodberry spell. WHICH IS FINE! I just changed my focus and we're having fun!

There's also the "gambler" background that has a REALLY powerful ability to basically just know if something is a good idea or not. This hasn't caused any problems, and everything is still going smoothly, but everytime I tell my Gambler whether or not something is a good idea, I get a sour taste and start thinking "am I giving too much info?". I want to make use of it and not say "mixed results" every time, but it seems hard to find the balance!

Are there any background abilities y'all have run into that are challenging? 

I am not sure whether any background feature is de-facto better than any other though it will depend on the campaign. I haven't found a group that likes to play a survival heavy game so finding enough found is rarely an issue. If the campaign involves a lot of travel by sea then yes a sailor background would appear to be useful, but if there ar eno sailors in the group the DM would ensure they have enough gold to pay for passage, in a landlocked campaign it might have no use at all. Most of the backgrounds allow for the DM to let all the characters feel their feature is useful if they want (When they stay at an inn they can do so for free if the performer does his stuff, when the party need to tell the king / meyor /whatever something important only the nobel might be allowed to talk to him and so on.

Regarding gambler RAW I would say it is one of the weakest features. It only applies to "During downtime activities that involve games of chance or figuring odds on the best plan".  The games of chance is easy to understand,  In theory this means that if playing blackjack the gambler knows what the odds of beating the dealer are whatever they are holding, the problem with this is the DM has to work it out meaning that such games have to be trivial (e.g. two players roll a dice and the one who scores highest wins) to prevent the DM having a math degree and spending 30min working out the optimum strategy for the games current position. "Figuring odds for the best plan" is somewhat looser but I would say it can only apply when enough data to calculate the odds are known. If the party are asked to kill a dragon but the quest giver doesn't know what type of dragon it is then you can not calculate odds, even if it is known to be a "young blue dragon" would the gambler know all the skills that the creature has and how hardy it is (the HP range).  If I was DMing a gambler I would probably only alow it to be used on games of chance and give them a small gain to their chances of winning, for example if a gambler plays blackjack and goes bust with a score of exactly 22 they can redraw that last card.

Quote from LeBattery >>

There's also the "gambler" background that has a REALLY powerful ability to basically just know if something is a good idea or not. This hasn't caused any problems, and everything is still going smoothly, but everytime I tell my Gambler whether or not something is a good idea, I get a sour taste and start thinking "am I giving too much info?".

That Gambler ability is only supposed to apply to "downtime activities", which gives you a great deal of latitude is deciding what it does and doesn't work on. It's just a limited form of augury, really

As for the question in general, I've always just seen them as RP opportunities more than anything else. Outlander's ability is fine... but it doesn't work in a desert, and what happens when there are more than six people in the group including NPCs? Who doesn't eat?

If your PCs are taking backgrounds just to get abilities as mini-cheat codes, I would have no problem nerfing them a bit either. For instance -- if three people have Outlander, that doesn't mean the ability has to 'stack.' Maybe an area only contains enough game and berries for max six people to eat for a day, regardless of how many Outlanders are out hunting and foraging

I’ve found that background abilities turn out to be useful at the strangest times. I recall once when a PC who took the Marine background from Saltmarsh used it to bring a ship to shore in a storm instead of wrecking the ship. I was completely blindsided and he successfully saved the party’s ship. 

Most of it is up to the dm and how best to ensure everyone has fun. In one campaign I took the faction agent background (harpers). From about level 6 their assistance included access to their network of teleportation circles so we could get between cities without the grind of overland travel.

I'm running three different groups through Rime, and (admittedly under my recommendation) always have my players pile on backgrounds focusing on survival (such as outlander and fisher). It has meant that the whole challenge of them finding food and water is all but eliminated, but if that was what they've chosen then I'm happy enough to give it to them. I maintain the emphisis on this being a harsh and difficult place to live by reminding them at the end of each day that their chosen skills has meant that they've been able to find small game to eat. Also, in choosing their abilities to survive well in the wilderness, they have of course forgone other useful background abilities. 

I had gotten blindsided a couple of times by players with unusual backgrounds cropping up in surprising moments though.