I waited 50 minutes for Nemo ride at 11:00am on Sunday of Labor Day weekend last year. Mostly it was stupidity. The posted wait had not been changed. Having never seen an actual line for Nemo, I didn't realize how long the line was until it would have been difficult to get out of it.
The days preceding and following Labor Day weekend are great. I can't compare it to other holidays because I haven't been to WDW on other holidays. My understanding is that compared to super crowded holidays, Labor Day weekend fares well. However, it is not EMPTY. I would say it is moderately crowded. We hopped over to MK after Epcot that Sunday. We were able to get FP for things like the Carpets, Buzz, IASW after our scheduled FP+. We watched shows while waiting for FP window to open up rather than getting in long standby lines. Then it rained, the park emptied out, and the rest of the evening the lines were a walk on - -I'd say from 8pm until closing. We saw the 2nd Electrical parade, 1st was canceled, so I guess the park closed at 12am.
For the past several years, the park hours at MK get extended on Labor Day weekend. Right now, closing is at 9pm. Last few years they added EMH evening hours to Saturday, and extended park hours on both Saturday and Sunday.