Linux sucks often comes down to, I am not used to it so therefore it sucks because I don't know it.  There are many reasons for this, which I just won't cover here, but there are also legitimate reasons to dislike using Linux, or to think it isn't as good as some of the alternatives.  To name a few of my personal ones...

Documentation: I will come back to this.

Plugins:
  While we have plugins, it is very difficult to tell exactly what is decent or isn't without spending a lot of time with it.  Some plugins sound great, but seem to crash my session fairly often with little warning.  You can say this is a problem on other platforms as well, and you would be correct to an extent when talking about plugins developed in the spare time of someone, or other 'free' solutions that aren't supported.  But I gotta say, I can't think of the last time one of my commercial plugins crashed a PT session.  Hell even running them through Wine with Festige they are pretty dang stable(I have had two instances I think where i had to restart everything, Jack, Festige, etc. as they weren't processing audio correctly, which may be as much Wine, Festige or any number of things as the plugins).  Many plugins sound horrible, but it is hard to identify these without going through each of the hundreds of plugins available.  And then you get those few gems that sound great and are stable.

Audio Hardware:
  Not just IO hardware, but that is huge as well.  And I don't mean to say this is the fault of the Linux Community, in many cases it is the manufacturers, straight up.  RME is good stuff, don't get me wrong, and I love my RME interface.  I also know that courtesy of FFADO etc. we have many more selections than we used to and have some solid mid range things, Focusrite and Echo Audio come to mind especially.  I also found that with little work I could get my Mackie Onyx-i series working if I cared to.  All well and good, but then we have things like Dante, which is picking up steam in the live sound world, especially for multitrack recording, and interfaces with a lot of live processing/mixers/etc.  Or we have things like a Metric Halo interface, or any number of interfaces coming out nowadays that have onboard DSP effects or dynamics processing.  The MH units are a great example because of their flexible nature, but even more basic like the onboard DSP on many Focusrite or Lexicon interfaces are not supported.  Or even more basic things like my Apogee Duet, which I see there are reports of it working that doesn't seem to be to certain, I have never bothered trying, or many interfaces without hardware controls that depend on software control we just don't have software for.  .  And then of course you get to the topic of things like Universal Audio's DSP cards, or the SSL Duende, etc.  All of these would be great for running commercial plugins on, except we can't use them of course.

Audio 'Drivers':
  This is still an issue, and it is an issue because the average user can't download Ubuntu and install Ardour, and have it run without having to set it up and tweak it.  This is in part due to the blessing and curse both of 'choice' in that for most of us here, we know standard Ubuntu probably isn't the best choice, or we know that we have to add realtime permissions to our user, or half a dozen other things.  But the average person from a different OS that doesn't know Linux is going to download Ubuntu, or Fedora, or whatever distribution they have heard of, or happen across first in a Google search.  Contrast this to OS X, user installs the OS, Download the app, and runs it, and all permissions, setup etc. are taken care of for them.  Or on Windows where without tweaking at least the damn stuff opens(Can't say that about the 'typical' Linux install with pro audio that depends on Jack as it is looking for realtime permissions) even if it doesn't run well.

Back to Documentation: 
  We lack a good single reference we can point people to for information.  linuxaudio.org is a start I suppose but I gotta say, if I go there thinking as a new user looking for information on what hardware works with Linux for audio, I could go to the site and have no freaking clue where to go from there.  It is set up much more for the people that already know Linux, as opposed to the people that want to know more about Linux Audio because they are interested in learning about it.  And to clarify, yes I personally know that under 'Resources' there are some good resources there, but as a new user, the typical kind I generally have to help in IRC, or elsewhere this would mean nothing.  And even after clicking that it means little looking at the list of things.  I am picking on Linuxaudio.org but that is just one example of many.

   On #Ardour we have known for a long time that good documentation is needed, but it wasn't till the past couple of years that something happened.  The FLOSS Manual is a strong start, but many people come to IRC before finding it.  What does that say when someone comes to IRC that knows nothing about Linux, and in many cases might know nothing about IRC, before they find the manual to a product?  What does it say when the most common question I see about Ardour is, does my hardware work with it?  The most common answer(That I myself give) yes so long as Jack works with it, or so long as your OS works with it.  I say this even though it means nothing to the people using the hardware.  We can send them to ALSA Matrix, or to FFADO, or tell them so long as their device is 'class-compliant' it will work, but damn, that is pretty painful compared to them just knowing if it will work or not form the manufacturer.  Even if they can find out if the device works, they have no idea HOW to get it working if the distribtuion doesn't do it for them, which leads to the second question, and on.  And what about devices that aren't listed?  We list devices that are known to work, but don't even address often devices that DON'T work, leading to lots of questions about devices that won't work with Linux just because they couldn't find a definitive answer anywhere that it wouldn't work.  Again I know FFADO does this to an extent certainly, but the ALSA Matrix for example doesn't, and we are still sending people to multiple sites to figure this out.

  These are just a couple of examples.  Documentation in general just needs to be rethought as a community I think.  But noone will ever be happy with it, so maybe I am just a grumpy old coot telling you youngins to get off my lawn as this has been gone around many times:)

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