I see some carbon frames with lugs and without. Is there a difference in durability or performance between the two types? I'm just curious because I'm looking for a new frame possibly.

Not really. Many of the "non lugged" cf frames have internal lugs.

When properly used, either technique can create high quality, reliable frames.

However, I will say that all of the CF frames that I have personally seen fail due to manufacturing defects were lugged (and the frame failed at a lug joint). Has anyone seen a monocoque frame fail, other than due to crash damage?

There is a difference. Read Craig Calfee's white paper (link below) Craig has been building carbon frames since 1987. Monocoque frames can have thin spots in some of the bends. The purpose for lugged construction is so you have uniform thickness in the tubes and they are thicker in the lugs (where most of the stresses occur).

Some may stay that Craig Calfee is biased and he might be but he build custom carbon frames and few companies can do that. He also builds a bike built for three that converts to a bike built for two, builds bikes from bamboo and repairs everyone's carbon frames.

I trust Craig's opinion on carbon frames more than anyone else.

Read his white paper:

https://www.calfeedesign.com/whitepaper1.htm

The frame on my bike is lugged and from what the manufacturer says, the front triangle is moncoque for lightness and the rear triangle is lugged for strength and stiffness where it matters. Whatever..

In most cases, it's more of a matter of: did the builder smooth over the lug joints? I would base the decision on test rides if you're deciding between frames. Try to forget the scale and what your eyeballs see.

Often, lugs are used to make it easier to tune the ride quality for each size of bike offered. For instance, larger diameter tubes on larger bikes. It also allows custom frame makers to tune specific attributes out of a given frame.

Lastly, smaller shops might prefer lugs rather than making different monocoque molds for each size, but that one is purely speculation on my part.