The only issue that hydracad had with large systems was that they used to have it set up so you were limited to drawing between 0,0 and 899',899' or something like that. I believe that has been resolved.
The only bugs I really had with HydraCAD involved the fabrication listing. It was just the loose material always seemed to give me fits. I could get it to list the fabricated pipe at near 100% accuracy, but the loose material was about 50%. You just have to draw in a manner that HydraCAD wants the information. It is not complicated and shouldn't really require you to change much of what you do.
The calcs work pretty good. It has 2 way data flow, but once you return sizes to the drawing, it was more of a pain to go back and change the calcs later if you needed to modify pipe sizes. You usually had to manually modify pipe sizing in the drawing at that point. It was not a major issue, just something to know. I am not very familiar with the new SMART Tools by hydracad that allows it to generate 3D drawings for import to NAVIS and other programs.
With regard to AS and Autocad based, you do import and export autocad drawings. I take all DWG files from the architect and import them to AS to draw. If needed, I can take my AS file and export to DWG and provide to anyone who needs it. It also works very well on large projects. I have done a 600k sq ft warehouse without issues, 200k sq ft hospital floors without drawing problems as well as large schools and other things like that. I am not saying AS is bug free and perfect by any means, but it is a really good program and does play well with AutoCAD. AS was the main drawing program for the City Center project in Las Vegas as well, so it can be used to handle monster sized projects.
I think you would be happy with HCAD or AS as both are excellent programs.
Travis Mack
MFP Design, LLC