I am playing BuildCraft on a cauldron server(hosted by me)with my friend. But every time I open the server, I get a message saying that BuildCraft was not tested on Cauldron servers and that it may cause lag. We would be happy if BuildCraft was compatible with Cauldron servers.

We would be happy if Cauldron didn't make far-reaching changes to the engine which break mods' expectations of the engine in ways which cannot currently be solved. We can only mitigate some of them.

Hint: Other mods don't warn you. They may, and probably will, often fail silently in subtle ways.