An app like
External 2 Internal SD will mount the SD card as "internal" storage, so you can have as much (currently) as 128GB of internal storage total. (There's a 200GB card out, but since it's going to retail for about $1,000, not too many people are going to be waiting on line to buy it. But a few years ago, the thought of 128GB in a microSD card was considered a sign of insanity. We
will see 1TB microSD cards for reasonable prices, just not this year.)
Otherwise, some apps just won't run from a card mounted as external storage (widgets won't) because - and I'm guessing, I didn't write Dalvik - of some particular class file they use. (And if a developer uses a class, it's because the app needs that class to function, it's not because he wants you to not be able to run the app from the SD card.) As Madd54 said, some parts of moveable apps can be moved - the app isn't installed to the SD card (Android doesn't work that way, it runs apps from what's mounted as "internal" storage), parts of it are moved, and links to those parts are left in internal storage, where Android can find them. If there are enough small parts that have to be moved, you lose space by moving the app.
I'm a software junkie - I have over 1,000 apps (some have more than 1 version) backed up and currently have 263 apps installed on my phone and enabled (and another 63 frozen) - all on internal storage - and still have13GB free on a Note 3 (32GB total). I rarely use most of the installed apps, will probably never use any of the frozen apps, but with 13GB free, I'm not worrying about freeing space yet.
But I don't have any media files on internal storage. All my ringtones, alarms and notifications are on the SD card. All my songs, pictures and videos are on the SD card. All the apps that have an option to store data to the external card are set to do that. And I have 26GB free on a 64GB card, even though I have 2 Safestrap slots on the card (which takes up a lot of space). If I were going to be on a 24 hour flight and I wanted to be watching movies through the flight, I'd probably buy a 128GB card and keep movies on it.
Move an app to the SD card? I've done it. It works - for those apps that can run from the SD card. But only to see if it worked. I can't see filling up 32GB of internal storage on a Note 4 with 128GB cards available. Not now. In 2020? Probably. We once thought a 500MB hard drive was all anyone could ever need. (No, Bill Gates never said that 640K was all the RAM anyone would ever need - that's urban legend.) But when you want to store a 16k feature-length movie on the card (which would be nonsense - VERY few people can see the difference between HD and 4k on a 6" screen, 8k video on a phone is more than just overkill, 16k video on a phone is totally ridiculous - but they'll do it if they can, because it sells phones), you're going to need a lot more card. And by that time, if you can still find 128GB cards, they'll probably be $5. (Try buying a new 2GB card today. You could 5 years ago, and they weren't that cheap for good ones.) And you're not limited to one card. Use an OTG cable and a card reader and you can easily carry a few TB in a shirt pocket and switch cards in a couple of seconds. (I have an old SD case that has 6 slots and can hold 4 adapters per slot - that's 24 adapters with microSD cards up to 128GB each in them. That's over 3TB in a case hardly larger than a pack of gum. Maybe not enough movies for a trip to the moon, but a LOT of storage for a phone.)