1. Expert marks can (and sometimes often are) be faked/forged. 2. Many advancements have been made in detecting faked/forged stamps in the last few decades. 3. The knowledgebase of faked/forged stamp has increased in recent decades.
For the reasons above, I typically place less value in old experts marks then I do with recent certifications. Don
The question is very broad and general. Especially without illustrations. It needs to be dealt with on a stamp by stamp basis. Whose mark is on what? Each expert had their own areas of knowledge. As you can see from the other replies and links - it all depends. Do you have a specific example to share/discuss?