Knots reduce the strength, ejection charges will degrade it over time, especially the really thin kind, etc. I've always, therefore, over-specified my kevlar. I use 100 or 150lb for LPR, but anything that's using an F or larger gets either 500lb or 1000lb depending on the weight. Small high power gets 1500lb.
I bought a lot of military surplus Kevlar years ago (many weights, 1000's of feet). After some failures with it I started doing a lot of research and came to the conclusion it really isn't very well suited for our purposes! Exactly for the reasons listed by many in this thread - the load limit is far less than the breaking limit, poor resistance to abrasion, it's not a fireproof as most believe, etc. The other big issue is that most knots weaken aramid based cord far more than traditional ropes - like nylon - and reduce the safety margin even more. The bowline knot for example can reduce the strength of kevlar by 50% or more, compared to about 30% for normal ropes. And according to a couple of studies I read, if kevlar is 'shock' loaded, it can suffer internal damage to the fibers that is very difficult to detect but can reduce the strength by a significant portion, leading to unexpected failure. Kevlar also hates any kind of a sharp bend - it will lose a significant portion of strength similar to a knot.
BUT, also as stated in this thread, it's very easy to get a 100:1 or greater margin with kevlar, and it's size and weight allow for easy packing and use in model rockets. Even 200lb kevlar easily fits in a model rocket that weighs less than a pound, and provides such a large margin of safety that a lot can go wrong before it will fail. So in that respect it works very well for rocketry. When using proper termination techniques and protecting the cord from direct flame and abrasion, kevlar can be a good choice for our use.
Unfortunately, most of us learn these lessons the hard way - there should be a '
everything you need to know about kevlar and rockets and don't want to learn the hard way' document that is required reading for all new rocketeers.
Tony
Unfortunately some of the links I used for kevlar comparisons are no longer working, but here's a post that shows how knots behave in two different kinds of ropes. In the chart linked to below, the graph shows the percentage of listed load that the know failed at - you'll see that Dyneema, which is similar to kevlar, simply does not like knots. With a bowline knot, it failed at 35% of the listed load limit! It also shows that a double fisherman's loop breaks at about 40% in dyneema, but is far stronger with marlowbraid, surviving until 82%. I realize neither cord listed is kevlar, but it's still instructive on how knots affect cord strength:
https://www.rocketryforum.com/threads/tying-knots-in-kevlar-cord.169359/page-2#post-2228022