We are using WFFM on 7.2 version and I see that we can still use SQL for WFFM in latest version. Are there any disadvantages if we migrate to latest version of Sitecore and still uses SQL?
This might be an opinionated question/answer, but I don't think there are real disadvantages.
Assuming you have xDB enabled (otherwise the answer is simple and you have to use sql) you should choose between analyticsFormsDataProvider (all in Mongo) or the combinedFormsDataProvider (analytics in Mongo, data in sql). Don't take the sqlFormsDataProvider as you won't have analytics.
By default the analytics is used as this is easy - everything is stored in the xDB, no need for extra sql databases.. But if you want to create your own reports/exports/... on the wffm data, most people will choose the combined as the sql is easier to use. With this configuration, the actual data is stored in a sql database and the analytics data (link between the interaction/contact and the form data) is stored in xDB.
It's all a matter of architecture - the decision should be made on what you want to do with your data (or possibly restrictions on where/how you have to save the data, e.g. security)