I have done 37 days birding in Thailand in two trips. My only birding in South East Asia so far & recorded 620 species there. Four Pittas, seven Hornbills, seven Broadbills, fourteen Owls, three Frogmouths, five Nightjars, twenty-three species of Phylloscopus Warbler (if you like that type of thing), Spoon-billed Sandpiper, Nordmann's Greenshank, etc. Keen to visit other places in South-east Asia but not sure I would place any other destination there ahead of it 'by far'.
A good gateway destination even if it lacks some endemic families.
All the best
Paul