Some VST plugins that I added to Davinci Resolve have problems. The GUI window is too small and I can access the cut out part of the GUI.
For example, all the VSTs from ReaPlugs: http://reaper.fm/reaplugs from Reaper have the same problems. However, they don't seem to have problems in other vst hosts. I experimented with VSTs from other companies and it worked ok.
I even used ResourceHacker http://www.angusj.com/resourcehacker/ to adjust the dialogue size but it doesn't affect. The only way to access the whole screen was setting the font size very small(like size of 4).
You're right, in Windows those Reaper plugins don't display properly in Resolve, though I believe you can still make adjustments to all parameters in the Inspector.
If you're looking for free plugins with workable GUIs in Resolve, try Marvel GEQ, TDR Kotelnikov, TDR nova, TDRVOS SlickEQ.
I don't think the problem for the majority of VST's will be fixed until BMD overhauls the entire VST system and adding HDPI support.
Resolve Studio 14.1 Windows 10 Fall Creator Update ROG CROSSHAIR VI HERO MB AMD 1800x 4Ghz 32GB RAM M.2 System Drive, M.2 Media Drive (All other drives SSDs) ASUS ROG-STRIX-RX580-O8G VC