Enigmatic flags mashup

Question

^{An entry in Fortnightly Topic Challenge #45: Flags}

The answer is a sentence.

Hint 0 before the bounty expires (the length of the words in the answer).

3,3,4

Hint 1:

As I said in the comments @Alaiko's answer is definitely on the right track, the problem is the method they use to compute the final answer. Using their method the answer would have a lot of letters while the true answer has only ten letters (see the previous hint)! Try to find a method that produce a ten-letter answer.

Hint 2:

There is something "strange" in @Alaiko's answer, it is about a mashup flag in the third row of the grid. After you fix that, it would be easy to find the way to compose a ten letters sentence out of the grid in their last spoiler tag.

Hint 3:

In their answer @Alaiko states that:
- For the first row, the trend seems to be that the first letters of one country also appear as the last letters of the other country.
- For the second row, the names of the two countries share the same first few letters.
- For the third row, the shared letters of the two countries either appear at the start of both countries or at the end of both countries.
Isn't the rule for the third row a bit strange? The sequence would sound more natural if the last rule was "For the third row, the names of the two countries share the same last few letters". Also, the only flag in the third row that follows the "shared prefix" rule is Latvia+Laos, that sounds more like an exception than a rule...

Answer 1

Alaiko did all the legwork and the OP gave pretty specific hints, so I don't want any credit for this, but it seems that if flag 2 is something like Yugoslavia (maybe these names are interchangeable? I'm not very familiar with European political history. Or maybe an older flag-I see at least one green Yugoslavia flag on the internet, or some other land starting with Y), we get:

YOU DID WELL

From:

Top row: (find the shared prefix/suffix combos)
Namibia + Ghana, Oman + Yugoslavia?, Peru + Russia, Benin + India, Andorra + Poland, Qatar + Argentina, Cyprus + Russia

Middle row: (find the shared prefixes)
Mongolia + Montenegro, Denmark + Ukraine, Burkina Faso + Burundi, Slovenia + Slovakia, Djibouti + Italy, Chile + China, Belarus + Belgium

Bottom row: (find the shared suffixes)
Wake Island + Estonia, Armenia + Estonia, Poland + Switzerland, Russia + Tunisia, Argentina + China, Pakistan + Afghanistan, Laos + Latvia

Thus...

The initials of the unused (not bolded) flags spell out OYDUDIWELL, or YOU DID WELL (3,3,4)

Answer 2

Partial Answer

The first thing to notice is that

the flags shown in the question are all combinations of 2 different countries/territories. The image below shows what the two countries/territories are that combine to form each flag in the puzzle.



Here is the same list provided in text form.

Top row:
Namibia + Ghana, Oman + Hungary, Peru + Russia, Benin + India, Andorra + Poland, Qatar + Argentina, Cyprus + Russia

Middle row:
Mongolia + Montenegro, Denmark + Ukraine, Burkina Faso + Burundi, Slovenia + Slovakia, Djibouti + Italy, Chile + China, Belarus + Belgium

Bottom row:
Wake Island + Estonia, Armenia + Estonia, Poland + Switzerland, Russia + Tunisia, Argentina + China, Pakistan + Afghanistan, Laos + Latvia

The second thing to notice is that

most of the two countries in each combination share some common letters in their names. Specifically, the shared letters either appear at the start of the name or the end of the name. The pattern of combination also appears to be different for each row.

For the first row, the trend seems to be that the first letters of one country also appear as the last letters of the other country. So, for example, in Andorra + Poland, and appears at the start of Andorra and at the end of Poland.

For the second row, the names of the two countries share the same first few letters. For example, Mon appears in both Mongolia + Montenegro.

For the third row, the shared letters of the two countries either appear at the start of both countries or at the end of both countries. For example, sia in Russia + Tunisia or La in Latvia + Laos.

The problem is that some of the countries in each row do not actually match this pattern. For example, Oman + Hungary in the first row does not share any letters at the front or back. It could be that this observation leads to a dead-end but it occurs for so many of the combinations that I doubt it is a coincidence. So, either I have misidentified some of the countries or there is something else that I am completely missing.

Assuming that I am on the right track,

I think that the next logical thing to do would be to extract the common letters from each pair of countries and use them to form a sentence. For the ones that have no common letters, I have placed a ?? instead. Doing so will give us:

NA ?? RU IN AND AR RUS
MON ?? BUR SLOV ?? CHI BEL
?? NIA LAND SIA INA ISTAN LA

While I can see some recognisable words like 'RUIN' and 'AND', most of it still remains cryptic. One other clue that I think might be important is the way the flags are combined for each one shown in the puzzle. For example, in Peru + Russia, the Russian Flag is combined in an 'L' shape with the Peru flag that is in a reverse 'P' shape. For Benin + India, it's different; one-third of the left side of the Benin flag is combined with two-thirds of the right side of the Indian flag. However, I am still not quite sure how important this is or how to make sense of it.