Hi Everyone would love to get your thoughts and ideas regarding using wordpress plugins and whether this is good or bad?

Scenario is a client wants to build an online business directory as well as have coupons similar to groupon.com i don't think that wordpress cms & plugins will handle this type of website especially in database size and growth... The developer currently quoting the client disagrees and says that it is okay to use wp and just use directory plugins and whatever else plugins it requires ...

Maybe i am just old school but i think it won't handle it in the long term? Would love to hear thoughts or suggestions... Thank you...