Actually the Lisa's emulation of the Mac was very very close. What it lacked was sound and this was due to the lack of hardware that supported proper sound.

There actually was a sound card for MacWorks, but I'm not sure how close it was to the Mac's.

Only applications that did not follow Apple's guidelines would fail on the Lisa. These usually did stupid things such as assume the size of the display. Those would also later fail on the Mac II's, or Mac's with updated display cards - such as that weird SCSI display card.

Speaking of which, there was a SCSI card for the Lisa from SunRemarketing.

If you had a Lisa with the SCSI and Sound options, you had a very capable Mac.

Also don't forget that a lot of the technology for the Mac came right out of the Lisa. For example QuickDraw. These machines are far more related than they're different.

The two big differences are that the Lisa was really a workstation, and unlike the Mac, it was open. That is you could expand it. The Mac was very much a closed system.