The main benefit (and reason for popularity) of the trust was that it allowed you to obtain an NFA item without having to obtain the signature of your local sheriff/CLEO and get finger prints/photos, etc. It was simpler and less intrusive of a process, and was your only option if you had an unfriendly local chief law enforcement officer.
The side benefit of the trust is that it was relatively easy to add other trustees so you could have multiple people who were authorized to use your NFA item.
There really isn't much of an advantage to a trust for estate planning or passing on an NFA item to your heirs. If you are an individual owner that can still be done through your estate as part of the normal probate process, and just requires some basic forms be filed by your heir with the ATF to transfer it into their name. I don't think they even need to pay a fee.
Any claims of legal protection by having a trust are dubious. You can bet that if something went sideways with the NFA item and there was a suit, they would sue both the trust and any individuals involved in the trust or the incident. Doesn't matter that a trust owned the item, they can still sue you the individual.
With the new NFA rules coming into play on July 13th, the main benefit of the trust is erased. Now no one needs CLEO sign off, and everyone (trust or individual) needs fingerprints/photos. Roadblocks are removed and hassle factor is about the same. There is still the ability of a trust to add new trustees post purchase, *however* the catch is that for any subsequent purchases all the trustees need to provide fingerprints/photos, so that hassle might outweigh the benefit.
Takeaway is that once the new ATF rules go into effect, you are going to see a *drastic* reduction in the number of purchases made using a trust. People will just buy as an individual since the process is the same and there's no need to pay to have a trust set up to avoid prints/photos/CLEO sign off anymore.