You often hear “You can’t be both beneficiary and sole trustee of a trust”. That is because, in such a case, the title is no longer split between two parties, one having legal and one having equitable title. If you have both, you have complete ownership. The trust dissolves. That is the only danger.
“Merger doctrine (trust law)
In the law of trusts the term “doctrine of merger” refers to the fusing of legal and equitable title in the event the same person becomes both the sole trustee and the sole beneficiary of a trust. In such a case, the trust is sometimes deemed to have terminated (with the result that the beneficiary owns the trust property outright).“