Boundaries is a very delicate topic, especially national boundaries. And even more in the south of Thailand.
Even when it’s tempting to add boundaries just to have them, I favor to let a group of Thai nationals decide what to import.
In this area remote mapper and foreigners can do great harm, even when coming with best intentions.
You are right: the broder is quite schematic. I corrected some stretches near Padang Besar, where good Bing images are available. But very often, the border is just somewhere in the jungle, and there are some places where both Thailand and Malaysia say that they do not exactly know where the border is…
Some stretches of the Malay–Thai border, especially those near inhabited areas, are walled/fenced. Where high resolution Bing images are available, it is possible to trace the borders from them with some accuracy. (I’ve refined the Padang Besar border a little, and done likewise for the area south of Betong.) The Ko-lok River also makes it possible to visually trace the eastern part of the border.
The remainder of the current border, which I understand were imported from the CIA World database, are low-resolution and can be quite inaccurate at higher zooms. Unfortunately, it seems to be the best free source available. The (unfortunately) ongoing insurgency notwithstanding, the Malaysian border isn’t anywhere nearly as contentious as that with Cambodia, so the main issue here is more a lack of available sources rather than potential disputes.