4 stars: Address of residence fields are a bit weird, and it's unfortunate that you can't enter special characters in any of the text fields, but if Spain is okay with the loss of data quality, so am I. Otherwise the application is quick and straightforward. It's also convenient that you don't have to give your exact address of stay in Spain.
3 stars: Very improvable UI. Most inputs won't accept regular or necessary characters (like whitespace for surnames, tildes or even underscore in e-mail). Sometimes the soft keyboard will cover the input. On country selection is very uncomfortable to scroll that much to get to your country, should not be a spinner. Some text won't be visible completely and fall out of the screen (and not scrollable)
4 stars: Works as intended. Only small issues: (1) Street address field does not accept hyphens (-), but addresses of apartment buildings typically need them. (2) The app is also somewhat slow to load when completing the form and when checking device compatibility.
3 stars: Good for most fields and as short as is practical. Only suggestions are 1) We have a hyphenated surname yet the software refuses it as it does the C/ abbreviation for Calle 2) took several attempts to accept my valid email address 3) required duplicated address input for our home address in Spain - necessary for visitors to Spain only.
5 stars: English is my second language but by this app health travel form was really easy to fill. No difficult questions, ...