I grafted 3 NMT varieties on my longan and they were growing good for 2 years, even had flowers and small fruits which i removed. Now only 1 surviving NMT on the longan and it hasn't grown much since the first year with fruits. I'm giving up on grafting lychee to longan, waste of time. I should have focused on grafting all the NMT varieties onto the big lychee tree I have. Now you can't get any lychee scionwood from the GRIN-germplasm in Hawaii.
I have no problem with any grafts on my lychee tree so I will stick to that. I only have one NMT vareity grafted on the lychee tree and it is still growing fine, but it grows very slow. I recently grafted some Sweetheart lychee from two sources (Ken, Son) and those grafts are doing well. In 2 yrs the Sweetheart, Maurituis (Frank), Seedless(Max) has grown about 12 inches. The largest growth was on the Sweetheart, some grafts are now 3 ft long. All have flowers and should have fruits this year.