I guess I haven't read any reviews of freedom pop, so I'm not sure what they are warning about, but I'll share my experience with them. Take it for what it's worth. I don't have any affiliation with them, aside from being a user, I'm not trying to sell anyone on using them, or dissuade anyone to not use them.
I added them to an old iPhone 5 two years ago, as a backup phone. We don't have a home phone & the price was right for very occasional usage. I don't recall the exact price, but the setup fee was somewhere between $10 & $30. For that, they gave me a SIM card with a local phone #, 200 free minutes a month & 500 megs of free data. IIRC, I believe that you have to enter your credit card number & when you hit the limits of the plan, they automatically charge your account for whatever the refill amount is. To make calls or receive texts, you use their app, rather than the iPhone phone & message apps (although if you log in with your apple ID, the messages app will still forward your texts from your iMessage account to the messages app on the freedom pop phone. I believe that the apple ID call forwarding works as well (not sure of the name, but the service where you can answer calls to your iPhone on your Mac or iPad-in this case, calls made to my real phone will ring onto my freedom pop phone, but not the other way around).
In the two years I've been using them (March 2017 will be 2 years), I have only paid a fee one time. I only use the phone about once a month, but it's convenient to have as a backup if someone needs to borrow a phone, my phone is doing an upgrade, or if I were to ever lose my phone, or anything like that. I never use the phone anywhere near enough to hit the data or call caps, and they have an app that tells you your data usage so you can monitor it if you are worried you're getting close. The app is hilarious though, as you log in, it shows you your data usage & then within 10 seconds, it crashes, every single time & it's not just me. it seems to happen to everyone.
Basically everything is an upsell with them, voicemail, I think texting, etc, and they use wifi as the primary method of calling, so if you are in wifi reception, your call will go over wifi. If not, it will use the Sprint network. Either way, the call counts against your free amount of minutes. Data over wifi doesn't count against you though, just cell data.
The one time I got burned (paid a fee) was when I opened the app & a popup came up to try a free trial of their international calling plan. I thought I clicked no, but apparently I clicked on try it, because a few weeks later I got billed $4.99 for the plan. It was easy enough to remove the plan after that & i never got charged again. I'm not sure if I would have been able to have them credit the fee back, but I figured that $5 for 2 years of service was not worth arguing about.
No way would I use them as my primary phone #, but if you have a spare phone laying around that you're using as an iPod, or a GPs or something and are looking to make it a little more useful without getting stuck with a monthly fee, I would say that my experience has been fine so far.
The service is pretty good on wifi, and a little less good on Cellular, but for free, I have no complaints.
In looking at the list of phones they support, it looks like they are about a generation behind. If you're still interested in using Freedompop, I'd bookmark their site & check it every couple of months to see if they've added the SE. I know when I started with them, they didn't yet support the 5S, but I now see it on their list of phones.
At one point, prior to having them on my iPhone 5, I had an account for a wireless hotspot through them. In that case, I actually bought the hotspot from them for somewhere around $100 & then got a few 100 megs of free data per month, but that got shut down when Sprint turned off their wimax network, and by that time, I had upgraded to a cellular iPad, so I had no need for the Freedompop service anymore, so I didn't upgrade the hotspot to an LTE version. Cancelling the service was no problem in that case.
