Your problem may be that you are not seeing what is on that planet. If some of your personel end up dying or capture is because the planet is well garrisoned. You need to make espionage (maybe on other planets) to get intell of the actual status of them. You can also hover a ship over that planet to see if there are troops there.

I usually set my sabotage and spy missions in garrisoned planets with 6 units, three of them being decoys. If there's a a character in the mission, it takes the place of two of those units.

Couple points the other replies haven't covered-

• An enemy fleet orbiting a planet you're trying to do sabotage/espionage missions adds an extra layer of difficulty for your success chance. (More ships = higher detection chance/ failure rate for your missions) Best way to handle this is to blockage the planet before attempting missions with valuable personnel.

• Enemy generals substantially increase the difficulty of your missions against planets/fleets with one assigned. You should target the general first with quite a few decoys, as noted by JapLance. I prefer to use disposable training center-recruited units instead of the much more valuable named personnel as decoys, as I believe they are prioritized for destruction/capture if the mission fails.

All of my advice is based upon many, many hours of playing the game, but if anyone else has noticed different results, please chime in. Sadly, there is no easy, mathematical formulay listed anywhere in the manual or files, so no one's being able to calculate the exact weights and effects of all this.

Use more decoys. A lot more decoys, especially on a well protected planet.

Correct me if I'm wrong, guys, but I believe the decoys are more likely to be captured/killed than the actual mission runners, correct? I've been placing my higher value characters on the mission itself so that if goes wrong, it's the disposable troops that are running decoy.

I'm late to this thread, but I also remember a few guides back in the day stating that the units assigned
to decoy should be the strongest. The idea is that the strongest unit will be a better decoy, and the weaker unit is more likely to slip past unnoticed.

Now, here are two VERY important pieces from the official paper manual (I still have it!) that should help:

Page 103 - Defectors:

"No matter what the mission type, a decoy's success in diverting attention from the agents depends on the decoy's espionage rating."
NOTE: Serving as a decoy on a successful mission does not enhance a character's abilities."