Mordekaiser received a rework since the previous answer was posted, which completely changed the way his ultimate works. Previously, it applied a damage over time effect that would create a clone pet of the target for Mordekaiser to command if the target died under its effects, and this could target and affect Shaco's clone.
The ultimate's description now reads:
R - Realm of Death - Cast time: 0.75sec
Mordekaiser slows the target enemy champion by 75% during the cast time, after which they're banished together with Mordekaiser to the Death Realm for 7 seconds.
Units between realms see each other as spirits, considering each other dead and negating any interactions between the two. Only the target and Mordekaiser will enter the realm; other champions cannot follow them. Everything that occurs inside the Death Realm is hidden, and all still inside are killed at the end.
For the same duration, Mordekaiser also consumes the target's soul, reducing their current attack damage, ability power, total attack speed, maximum health, armor, magic resist, and size by 10%, in addition to gaining them for himself. If the target dies while inside, Mordekaiser keeps their partial stats until they respawn.
If one of the two affected champions leave the Death Realm, because of having died or having it dispelled, the other will do so as well, but the stats will remain stolen for the full duration.
Shaco's clone, while considered a "champion" for the purposes of most abilities, is not affected by Mordekaiser's ultimate ability now. Mordekaiser will perform the cast animation, but both he, and the clone, will not teleport to the Death Realm.
Here's a gif, taken from this video, highlighting what happens when this interaction occurs:
Not pictured is that this does put Mordekaiser's ultimate on cooldown, so it is not beneficial whatsoever for Mordekaiser to use his ultimate on a Shaco clone.
Short answer: Morgana can stop a Katarina with the stun off of her ultimate but not her snare. Ryze cannot stop a channeled ability. Maokai can use his knock up to stop channeled abilities.
Snares and roots are considered movement modifiers. They cause the target to "halt movement entirely, whilst still allowing auto attacks and abilities that do not require movements." Source (For further reference here is a full list of all the CC types and their effects).
Further information about channeling spells:
All channeled abilities can be interrupted by enemy champions using any "interrupt" crowd control effect on the channeling champion. Interrupts do not include slows, blinds, or snares. They can also be interrupted by the player by giving orders such as walking away or activating another ability. Using most summoner spells will not interrupt channeled abilities.
Source
Best Answer
It's totally random. I tend to place it in areas that are congested, like tri-bush area in jungle, the paths close to the blue on each respective side, near the wraiths, wherever there's a teamfight. Try to actively see where your jungler is, and use it when he ganks for easy assists. Also if I plan on diving my enemy I place it between him and his path to his tower.
This is anecdotal, however in response to your OP, yes it is unreliable.
EDIT: you may or may not play these champions, but if you play Karthus, Soraka, or Shen, you should have a pretty good understanding of how to watch the mini-map and be a good global presence. Use GP's Ult in similar situations that you'd use theirs. (I.E. try to secure kills or score a few easy assists, even if it doesn't turn the tide of the teamfight, you'll get cash if an enemy dies and you don't have to leave lane.)