I have been with Smarty since May and usually travel 2-3 times per month mainly to Germany, Portugal and Spain. I can say that experience with Smarty on roaming has not been great, which is how I ended up on this discussion as I was searching options. That said experience with EE previously over several years was exactly the same. I didn't have to change ADN with EE but always had to switch to manual network selection and toggle between them trying to find one that works. This is tedious as toggling between manual and automatic itself takes several minutes and then you need to try each network which takes a few more minutes. Not sure if anyone has found way to do this faster.
I would assume local subscribers will always get priority in countries which would also have an impact. I would also assume contracts between providers vary thus why some say that their companions have better experience in different countries. Side note but I assumed same thing when I couldn't get Smarty to connect to data in central London as would assume 3 (I think that is Smarty base network) subscribers would get priority in congested areas. What it's hard to find is any research on which provider has the best actual connectivity when roaming, it's always just comparisons of cost and options.
A potential solution I have seen mentioned elsewhere which is what I was researching when ended up here is getting a second sim from different provider and keeping both active. Hypothetically the iphone will switch between whichever one has data connection, I remain skeptical this will happen automatically but at least would increase chances of one working for maintaining connection. Getting one EU one to combine with Smarty could then be an option. Obviously it's higher monthly cost maintaining two but might be worth it if it's less frustration.
In summary it's frustrating and is a problem but it doesn't seem to be just a Smarty problem. I have also always been able to get a data connection with Smarty roaming, it's just a matter of how long it takes fiddling with settings to get there and if I get the connection before I throw the phone out of frustration 🙂