BuildCraft machines don't connect to Universal Cable, Forestry machines do.
Forestry build 30, Mekanism build 75, BuildCraft 6.0.17

Cable still doesn't connect to BuildCraft machines, but it does connect to Forestry machines.

Minecraft: 1.7.10
Mekanism: 8.1.5 (All addons have the same version)
Forestry: 3.6.2.19
BuildCraft: 7.0.1.3
Forge: 10.13.4.1472-1.7.10

Buildcraft machines also don't receive any power from an adjacent and properly oriented energy cube. Wooden kinesis pipes will pull power from an energy cube properly, though.

Also, universal cables connect to kinesis pipes, but only if the cable is placed after the pipe; they won't connect if the cable is placed first. The pipes won't transmit any power, but a wooden pipe will slowly drain power without transmitting any. Breaking the cable after connecting it causes a crash, but the stacktrace points to Buildcraft as the cause rather than Mekanism.

Yep, I had the exact same thing. I had a filler connected to a diamond kinesis pipe, connected to some universal cable, connected to an energy cube. The crash occurred when I removed the universal cable with a pickaxe.
From what I can understand of the crash log, the kinesis pipe tried to draw energy from the cable, but you'd already removed the object, causing a null pointer. I may be wrong about all of this, but would it be that on destruction of the cable, you need to tell the kinesis pipe that you're no longer connected?
I wish I knew Java/Forge a bit better so that I wasn't shooting in the dark.

A link to a bug report: http://pastebin.com/tj1tAfjZ

It seems that using a wooden kinesis pipe and then a golden pipe connected directly to a cube works. Using universal cables however will not work with buildcraft machines.

Should be fixed now.

the cables still dont connect for me. buildcraft 6.0.17/ mekanism 7.0.0.87

@TurtleGalaxis it is fixed in the Mekanism 8 branch. You can compile it yourself,
but be cautious with all the new stuff!

where is the download for mekanism 8? or am i blind and it's on the normal download page?

You'll need to compile it yourself if you want to use it, it's considered a pre-dev level of unstable so it's not being built by any servers as far as I know.

cautious? as in bugs?

Cautious as in there's a lot of new work-in-progress stuff that isn't finished yet and very probably bugs, yes. Also at various times things in various stages of not-finished that will cause crashes, though we try to avoid that.