Can you trust a survey??
Usually you can trust a survey if you know what to expect out of it and you use a little common sense. A surveyor does not have Xray vision and can''t disassemble and examine the internals of things but a surveyor should be able to examine what can be seen and touched and smelled and make reasonable educated guesses at what is might be happening. If you do a seatrial the surveyor can take guesses at the engine condition and if the boat is the water a surveyor can evaluate how well seacocks move but maybe not if they close completely.
To a great extent using a surveyor you can only find out what is reasonable to find out. But that is often way more than you as an individual can find out on your own.