Some varieties of walnut do not react favorably to "stabilizing". The other part of the equation is that the term "stabilizing" is often loosely thrown around...... IE: I've heard some folks claim they are stabilizing by soaking it some concoction.... when in reality stabilizing in much more involved. That being said, IF you choose to have the wood stabilized, make sure you send it out to a professional such as K&G, Texas Knifemakers supply, or WSSI. I personally experimented with doing my own stabilizing for a couple of years..... its FAR more economical to just send it to a professional.
Walnut, being what it is, requires some type of protection for longevity as a knife handle. My personally choice when stabilizing isn't an option is to use Tru-Oil. Its the stuff you can find and gun shops and even Walmart for finishing/refinishing gun stocks. It requires some time and effort to apply correctly, but it does offer a durable, long lasting finish to raw woods.
You mentioned that handles have separated from the tang(s)... There could be a lot of factors coming into play with that...something as simple as taking some walnut that has been curing for 50 years, then using it around moisture/water can cause the wood to "swell" or "warp", which will often cause separation from a tang. IF those factors are not involved, it's likely a combination of preparation issues, and "glue" issue. If you use/used the two part, 5-Minute epoxy that's available in most stores, chances are good that it's not waterproof, or much of anything "proof". Even going to the "2 ton" over the 5 minute is a step up. There are a number of much higher quality epoxy type adhesives available then either of those. I personally use Brownells AcraGlass, but any of the higher end epoxy type "glues" have superior performance to the "hardware store" epoxies.
Obviously I can't say with certainty that those issues are your culprits, but based on past experiences, those are generally contributing factors.
Another big thing is the preparation of the mating surfaces....... flat, true, light "roughed" up, and CLEAN. Two of the biggest mistakes generally made when using any epoxy type adhesives..... 1. Improper surface preparation. 2. OVER clamping...meaning that the clamping pressure is to intense, and the majority of the "glue" is forced out of the joint, which causes premature joint failure.
If you're creating a full tang knife (one where you use "slabs" glued directly to the tang) a trick is that once the tang and handle slabs are flat and true.....use a contact wheel to grind a "hollow" down the center of the tang....this gives that glue someplace to abide, versus having it all forced out of the joint when the slabs are clamped onto the tang.