How can I make it so that an otherwise colonizable planet (or moon) can (entirely naturally) seem uninhabitable from a certain point on its surface?
Background: There are a series of immovable gates scattered across the galaxy by an unknown race. Each gate has access to thousands (at minimum) of other gates; enough that there is no danger of running out of new planets anytime soon. A majority of these gates are on planets, moons, and asteroids. These gates are used by other races to colonize and exploit planets. Just about any environment that a human in a space suit could survive in is a good candidate for colonization. Exciting or valuable planets are snapped up by corporations or polities, while less interesting planets are noted and eventually sold to other groups. Planets with extreme conditions or that present obvious hazards are passed over and blacklisted, and unless there is reason to do so, are generally not explored far beyond the gate. A robotic civilization examines planets on this blacklist in greater detail, looking to colonize planets that not as uninhabitable as they seem to be. Planets like this would seem to be bad candidates for colonization or mining within a ~100-kilometer radius from the gate, but would be more habitable beyond that. Ideally no more than a quarter of the planet would be completely infeasible to colonize.
I'm looking for almost any sort of hazard that would make a planet (incorrectly) seem dangerous or hostile enough to someone stepping through one of these gates that it's inadvisable or not worth the effort to exploit, with three stipulations:
1. Nothing interesting: There can't be anything that would make mining or research seem to be worth the risk. Obvious signs of abundant rare elements, scientific anomalies, alien civilization (no matter how primitive), or native life would all be too interesting. I want anyone who discovers that the planet is inhabited to wonder why the inhabitants bothered. Planets with really unusual dangers are cool, but I want explorers to be saying "Wow, this place sucks!" rather than "Wow, I wonder what's up with this place!"
2. Gate access: The inhabitants need to be able to occasionally access the gate, both for initial colonization and for import/export purposes. The gate being inaccessible a majority of the time would work, as would having gate conditions be lethal within a matter of hours. Sticking the gate in the middle of an ocean of lava would not.
3. Novel: I already have a tidally-locked planet with the gate on the terminator, which discourages colonization with high winds, ash storms, and the occasional rain of semi-molten rock carried over from the hot side. Weather-related answers are fine as long as they're dissimilar enough from this. I also have a planet in a white dwarf system where the gate receives enough ultraviolet radiation to destroy even hardened electronics in a matter of hours.
I have a few ideas of how this could work.
Dangerous system: A planetary system that appears to be on the verge of suddenly planetary catastrophe would not be worth doing anything with unless there was something worth the risk. A star that looks like it's about to go nova, a large asteroid belt likely to send asteroids at the planet, or an x-ray binary likely to sterilize the planet would all make potential colonists turn and run. The difficulty would be how to make it quickly obvious that the planetary system would probably kill you, while allowing further research to conclude that it won't.
Volcanic activity: Frequent volcanism can rearrange landscapes and kill without warning. Given time, it could be studied and predicted, but on an otherwise unremarkable planet, that generally wouldn't be worth the effort. Unfortunately, it would be hard to limit the volcanism such that it's intense enough in certain areas to scare off explorers, but mild enough elsewhere that permanent structures can be built. Additionally, major volcanism would be likely to bury the gate in rock, as well as potentially making surface conditions too similar to those of Venus.
Planetary purge: Some frequent event that, while apparently unpredictable, can be predicted and allow the inhabitants to bunker down to survive it without much issue.
An ideal answer would include an explanation that passes a reality check, but if necessary I can just make a follow-up question to ask how it could be made to work.
Edit: Made it more obvious that I'm looking for an apparently dangerous planet, not just an uninteresting one.
Edit2: To clear up confusion, 'colonizable' means the following:
- Temperatures between -250° and 200° C.
- Surface pressure ranging from vacuum to 3 atms.
- Gravity at or below 2.5g.
- No conditions that would frequently destroy buildings dug into the crust.
- At least 75% of the time, external conditions wouldn't cripple or kill a human in a hardened space suit.
- Flooding, tides, or underwater land aren't an issue unless the liquid would be hazardous long-term to a deep-sea submersible.
- The presence of enough metals and carbon for at least low-scale industry, and enough power options (solar, geothermal, volatiles, fusion, etc.) to support a colony.
Edit3: Again, 'colonizable' is a relative term. Habitability for unaugmented humans without extensive technological support isn't a concern. If a Mars rover can trundle around on the surface of the planet without summary destruction, consider it to be well within the bounds of colonizable.
Edit4: In response to a question. The gates are two-way and have been demonstrated to be able to reassemble in a matter of weeks from anything short of the detonation of an antimatter bomb. There have even been (inaccessible) gates that appear to be located deep in the atmospheres of gas giants. More information on the gates can be found here and here.
Edit5: Please do not have your answer involve aliens in any way. Please do not have your answer involve life in any way. Please do not have your answer make the planet unusual enough that either native life or alien intervention are the most likely explanation for how it came to be that way. The only exception is if it would take extended and in-depth exploration and research on the planet to figure out that that the conditions didn't arise naturally