Is it more difficult to publish a paper that demands reviewers with knowledge from many different areas of mathematics?
No, in my opinion as a (midcareer verging on senior) mathematician, there is no such thing as "too many new ideas to be published at once".  This is a curious reaction.

Whenever a student asks a "My advisor said....[something strange]" question here (or elsewhere), I have to wonder how deeply to engage in the possibility that the student somehow misunderstood the advisor.  Unfortunately misunderstandings between students and advisors are amazingly common...even when both parties are "good people" in every sense of the word.  More than a decade after graduating, I still remember spending several weeks of hard work on certain things my advisor asked me to do.  When at last I would go back explaining how hopeless it seemed, it most often turned out that there was some miscommunication: I didn't work on the problem that he had intended, or I hadn't read the right paper. [I remember wading through Katz's Rigid Local Systems because of a suggestion made by my postdoctoral supervisor.  Being a postdoc I was more savvy than a PhD student, so I probably spent at most a week before I went back to say "Really?  This is relevant to what I'm doing??" only to learn that, no, that was not the book of Katz he was talking about.  Still, I'd like to have that week back...]

I would encourage you to consider the comment "the draft is good, albeit difficult to understand".  I'm guessing that is meant to be independent information from the remark that your paper contains "too many new ideas".  Some key questions:


Was the draft prohibitively difficult for your advisor to understand?  People [students, advisors, referees...] say that writing is hard to understand in two very different circumstances: they may literally be criticizing the writing style.  But it is also quite likely that they are trying to say -- in a way which saves face for them and for you, but is in fact rather unhelpful because of that -- that they gave up before they could tell whether your arguments were correct, and they would only be willing to put more time into a better-written draft.  You deserve to know whether your advisor vouches for your work.


Related:


Does your advisor have the expertise in the disparate areas you are pulling together?  If not, he is really not the right person to be asking about this.  People sometimes seem to think (or more likely, to hope) that if they have a paper on "number theory" then I will understand it, and that if the paper references some work or lecture notes of mine then I will really understand it.  No way.  I often receive papers to referee which are about several things at once, one of which is part of my expertise but one or more isn't.  I like to learn new things, so I'll stretch to a certain point, but beyond that I just decline to referee the paper on the grounds that I'm not qualified.  If I can, I direct the editor to someone who is qualified; if I can't, I apologize for not doing that, but in no case do I intimate that no one is qualified to referee the paper.  How could I know that?!?


The idea that your paper is simply too ambitious is really a poor one.  At most it means it is too ambitious for your local mathematical community, and if you hear that as a reaction to your work which is otherwise said to be "good", it's a sign that you need to find a larger pond.  


  I am convinced that people will find the result noteworthy and interesting, but I also agree that the combination demands (basic) knowledge from many different mathematical areas.


Then you should find people who feel that the result is noteworthy and interesting, ideally those who understand all of it, but even those who understand some of it and can be supportive could be helpful.  If you think your paper is correct and reasonably (even if not perfectly) well-written, why not submit it to the arxiv?  If your paper draws together several different things, then try showing it -- via emails, for instance -- to people wom you know to be experts in at least one of those things.  They can tell you whether they are qualified to understand the entire paper, and if not they can (perhaps) tell you who is.


  It is a disturbing but possibly realistic perspective that researchers keep the same old soup at low temperature on the cooker. But isn't that too pessimistic? There are so many mini results published, why should I cut down on at least trying?


Yes, that is too pessimistic.  If you've done something valuable and technically difficult, it will be publishable.  The level of technical difficulty may make the reviewing process more lengthy (it should; you are aware that you don't want rubber stamp reviews of your math papers, right?).  On the other hand, in mathematics "technical difficulty" can be a selling point: if you have done something that is broadly valuable but that few people (or no one but you) would have had the acumen to pull off, then you have done something very impressive and valuable indeed.

Good luck.