As many times as I have spammed energy siphon, I have never seen it have any effect on an enemies subsystems. I will have to better test it on a player for verification, but at a 25% chance I have not ever seen an enemy npc lose any subsystem abilities for 5secs ever.
I have no idea about the effect on NPCs, but I can confirm it works on other players. My problem:
You slot one:
You disable 1 subsystem 25% of the time
You slot two:
You still disable 1 subsystem only 25% of the time
You slot 3:
You get the point, same as 1 and 2.
Now, the chance of all three NOT procing on separate rolls is about 42% (75%^3). So I should see it proc at least half the time with 3 doffs. If not that, then it should disable more than one system (up to 3) depending on how many you have slotted. This doff is broken and I've been complaining for a while about it here:
When it does appear, it will appear as a phaser proc. If you're using phasers then you might not notice it.
I use romulan plasma, and everytime I use this skill w/doff on enemies it shows the siphon symbol, but has never once shown a phaser subsystem (or for that matter any subsystem attack symbol) proc. If it does in fact work out of all the countless times I have used it, the only thing it possibly knocked out is their aux or wp. Yes I do monitor every buff, and debuff applied to my tested enemies and not once have I ever seen it apply a debuff to npc's. At 25% chance it should proc fairly often!!! As of yet I have not tested it on pc's, so I will have to take your word till I can.
Ok after repetitive uses it finally showed its ugly head that it does work on npc's, however I will add after some 50 uses it only proc 4 times giving what I would say is more on the grounds of a 5% chance of proc a subsystem offline chance, and not the 25% stated.
EDIT: Also noted it also seem useless when used against most npc's due to most of the time it occurs they simply clear it with an emerg2Xsystem to fix it.