There is no limitation on the admob object or any of the monenanization features - they are available even in the free version.
But don't forget that there is a limitation that free users can't publish commercial products. If they want to make money in any way with a Construct 2 made game they have to get a personal license.
Using Clickteam's tools I sometimes get unexpected behavior when I try to put too many actions on the same event line. For example, on a Start of Frame event if you try to read data from an array and then use that data there can be issues. So you have to create another Start of Frame event and put the new actions there. Is this a problem in C2, as well?
I haven't seen an issue like this with Construct 2. You can put as many actions next to an event as you want.