People always post negatives on the net, rarely positives.
Do as others suggest, test drive one for yourself.
As with any modern car, as long as it has a service history and you get at least 12mths warranty with it you should do ok.
No manufacturer has a problem free car, there are good and bad from all makes.
The advantage of buying a Megane is that they are not uncommon so you can aford to be choosy and get the model/spec etc you want.
The Laguna gets a lousy review as well. There are far too many Meganes & Lagunas on the road for them all to be bad.
We habe both a Laguna & a Megane and are pleased with them both.