@ JogoReal and Elvis_4720 - What have your results been like when your opponent has played 2...c6? This seems to be the real test for this opening, as discussed above.
Portuguese Opening: 1. e4, e5 2. Bb5

@ Karpark
After 1. e4 e5 2. Bb5
Move Frequency Score Draw
1: c6 896: 41.3% 69.2% 5% -0.14
2: Nf6 450: 20.7% 74.2% 5%
3: Bc5 221: 10.2% 77.6% 3% +0.08
4: a6 214: 9.8% 67.7% 4% +0.01
5: Nc6 200: 9.2% 70.0% 7%
6: Qg5 53: 2.4% 60.3% 0%
7: Qh4 46: 2.1% 63.0% 9%
8: f5 19: 0.8% 60.5% 5%
9: Qf6 11: 0.5% 95.4% 9%
10: Be7 10: 0.4% 90.0% 0%
11: Bb4 9: 0.4% 88.8% 0%
12: Bd6 8: 0.3% 43.7% 0%
13: Qe7 5: 0.2% 80.0% 0%
14: Ne7 5: 0.2% 100.0% 0% +0.50
15: a5 3: 0.1% 33.3% 0%
16: c5 3: 0.1% 100.0% 0%
17: h6 3: 0.1% 100.0% 0%
18: b6 2: 0.0% 100.0% 0%
19: f6 2: 0.0% 100.0% 0%
20: Nh6 2: 0.0% 100.0% 0%
21: g6 2: 0.0% 100.0% 0%
22: h5 1: 0.0% 100.0% 0%
23: Na6 1: 0.0% 50.0% 100%
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TOTAL: 2166:100.0% 71.0% 5%
Note: Exported from a "Scid vs PC" tree with mask, some computer evaluations are added by me as comments.

Now after 1. e4 e5 2. Bb5 c6 3. Ba4
Move Frequency Score Draw
1: Nf6 325: 36.2% 67.3% 5%
2: b5 208: 23.1% 78.6% 5% +0.24
3: d5 204: 22.7% 62.5% 7% -0.34
4: Bc5 49: 5.4% 65.3% 4%
5: d6 31: 3.4% 87.0% 0%
6: Qg5 25: 2.7% 50.0% 4%
7: Na6 12: 1.3% 62.5% 17%
8: Qa5 9: 1.0% 77.7% 0%
9: a5 8: 0.8% 75.0% 0%
10: f5 6: 0.6% 41.6% 0%
11: Ne7 4: 0.4% 100.0% 0%
12: a6 4: 0.4% 100.0% 0%
13: Qh4 2: 0.2% 50.0% 0%
14: Nh6 2: 0.2% 100.0% 0%
15: Qf6 1: 0.1% 100.0% 0%
16: Bd6 1: 0.1% 100.0% 0%
17: Bb4 1: 0.1% 0.0% 0%
18: Be7 1: 0.1% 100.0% 0%
19: Qb6 1: 0.1% 100.0% 0%
20: [end] 1: 0.1% 100.0% 0%
21: b6 1: 0.1% 0.0% 0%
22: g6 1: 0.1% 100.0% 0%
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TOTAL: 897:100.0% 69.2% 5%

Now after 1. e4 e5 2. Bb5 c6 3. Ba4 d5
Move Frequency Score Draw
1: Nc3 199: 97.5% 62.0% 7% -0.66
2: exd5 3: 1.4% 66.6% 0% -0.22
3: Qe2 2: 0.9% 100.0% 0%
___________________________________________
TOTAL: 204:100.0% 62.5% 7%

After 1. e4 e5 2. Bb5 c6 blacks have a slight advantage. Being a rare opening if whites know it better than blacks (as usual) this can compensate a bit.

Thanks. I trust your knowledge and experience as, unlike you, I've never played the Portuguese Opening. However I think I'll stick with 2. Nf3 as white for now!

Still, 2...Nf6 first makes perfect sense. What's so big about 3.d4? Black is more than fine after 3...c6 4.dxe5 Nxe4 5.Bd3 Nc5.

Sure, that line is good and solid for blacks. My Stockfish evaluate it as -0.14. It is not the traditional main line. Main line is 3...Nxe4, and not so good in my view.

The value of the opening comes when you put the opponent playing with blacks an opening he don't know and an opening that, like all open games, is rich in traps and tactics. If you follow the best lines by the computer it can easily get a small advantage for the blacks. Anyway computers almost never play "The Portuguese".

The problem with moves like 2. Bb5 is that they commit pieces very early, letting the opponent easily adjust to the circumstances. What business does the bishop have on b5? Usually the Bb5 move is made to pin the c6 knight and, as such, to put more pressure on the e5 pawn. But here, the knight isn't on c6 yet... So why do it? Just to develop the bishop randomly? After eventual ...c6, the bishop will have to drop back, and black will be able to continue with ...d5, gaining a strong center.

2.Bb5 is an obvious mistake that is why it not even a named opening, it is a blunder.
Come on, it's not even an inaccuracy.

Considering computer analysis evaluations, after 1. e4 e5, 2. Bb5 is not objectively worse than 2. f4, the move of the King's Gambit. Maybe the books should move its name to "King's Bishop Pawn Blunder", so...
"2.Bb5 is an obvious mistake that is why it not even a named opening, it is a blunder."

I mean sure it's playable as anything else. I don't like playing against 2...Nf6 which seems to just ignore White's ideas and work out in the end. 3.d3 and 3.Qe2 look interesting to me and aren't the main moves but I think 3.d4 just leads to a typical middlegame that isn't all that surprising to anyone.

I mean sure it's playable as anything else. I don't like playing against 2...Nf6 which seems to just ignore White's ideas and work out in the end. 3.d3 and 3.Qe2 look interesting to me and aren't the main moves but I think 3.d4 just leads to a typical middlegame that isn't all that surprising to anyone.
What "typical middle game" is that after 1.e4 e5 2.Bb5 Nf6 3.d4? Can you show us a game?

I mean sure it's playable as anything else. I don't like playing against 2...Nf6 which seems to just ignore White's ideas and work out in the end. 3.d3 and 3.Qe2 look interesting to me and aren't the main moves but I think 3.d4 just leads to a typical middlegame that isn't all that surprising to anyone.
What "typical middle game" is that after 1.e4 e5 2.Bb5 Nf6 3.d4? Can you show us a game?
This doesn't look like difficult play to me and there are similar positions in many other king's pawn openings.
2166 games in my database where I did play the "Portuguese". Quite some experience.
After 1. e4 e5 2. Bb5 the best moves for blacks are 2...c6 & 2...a6. 2...Nf6 is doubtfull cause of 3. d4 called originally Gambito Portuense (Oporto Gambit) mentioned usually as Portuguese Gambit. 2...Nc6 is mostly good but weird opening positions may arrive when whites don't transpose to the Spanish by 3. Nf3, and whites usually know those positions better.