I have an 80/channel AVR and wants to connect a center speaker which only 75 watts. Is this ok? Any disadvantages?

Hmm...same question twice in one day.

Bottom line...no, you're fine. Generally the more power, the better. You'll damage your speakers far quicker by under powering them.

unless you BLAST the volume, your speakers are perfectly fine.

power ratings are very subjective as they are generally guidelines or potential power ratings and not actual truths, especially with amps.