Hi, Wprins, Yes i'm using Firemonkey, It has evolved a lot since XE5, It's more stable, fast and reliable.
For backend I use a rest architecture, webservices created using Php. I don't use clientdataset or fireDac or whatever data access components, just Rest staleless webservices. So its a multitier architecture.
What is necessary to keep in mind is that mobile is not the same as desktop, we must find ways to keep the app fast and avoid to use too mush components in a single form, also the use of frames to insert forms in a main parent form is a must, have severals forms open in memory can just exauts the mobile device resources.
Anyway we can also build great mobile apps using Unigui, I have made quite some mobile app with unigui, of course not native, running via browser but with good results.