Situations could be:
- your map is really bad and you need to keep map control, so you can’t stop producing full army
- opponent has bed civ and you want to keep up the pressure
- opponent has much better lategame civ and you want to win early
- opponent goes up to castle faster and you can’t let them get map control/faster boom so you keep attacking and building army
- you’re DauT and want to have fun
I would presume we are talking about some scout + ranged army feudal warfare, however it came about. Whether it was from a typical 20-22 pop scout build or something from an oldschool forward (maybe, but this is certainly not really recommended these days) and it transitioned into this.
A general theme on how full feudal is most powerful is correctly evaluating strategic points on Arabia and making these work toward your benefit (pretty sure this is only map is works on, aside from Arabia-like maps). Strategic points here being where the main gold is placed which would be infront of his base (secondary gold placements can come into play as well; this is the only real resource you should take into account when considering full feudal), whether there are hills overseeing opponents base (or by his gold as well is what I'd look at instantly), and openness. What you really want for this to win around 9/10 times is main gold infront of his base and him being open. And if you somehow have all 3 of these, its definitely worth a shot.
The last one, openness, is very important to evaluate correctly. If opponent is open, that means there are multiple spots to hit with your army. Say you can't hit his gold right now because he has a tower or he has enough army to stop you here, you should then be looking for other areas like where mill or wood is (you can also wait and buildup army close by during this timeframe, but better beware of him pulling a quick Castle Age). If there are hills nearby here, you take advantage of those. Then later you can go back to main gold once your army is built up.
Also, with openness, you need to be able to foresee that your opponent won't be able to close off spots, mostly key areas. This means they'll have difficulty defending a feudal onslaught if they can't close off an important area like where their main gold is located. That can be tricky sometimes, as there is an old game in my head vs slam I'll always remember, where it was something like he had a 'very' badly placed main gold and a hill nearby. I misevaluated it entirely, thinking slam wouldn't be able to close this off and defend. Basically, he placed his ranges by gold in such a way (with a tower) that it blocked the entire hill and I was never able to get on the hill without perhaps, big army losses, whereas he was able to camp there up until hitting Castle Age much quicker than me. This foresightful mistake contributed to a rather difficult game that sadly wound up dcing due to voobly crashing for both of us. You don't want to make this mistake especially if opponent is trying to Castle out of your attack, as I was surprised how useless my attack was (he also had one wood behind here, so the semi-openness didn't matter as I couldn't attack here).
A last thing to mention is the time you initiate your first attack (this matters a lot). Generally, you want to be doing your first attack around 16th minute. 17 is doable as well, any later and this is outside optimal feudal play. The reason this timeframe is so critical is that if your opponent planned some 21-22 minute Castle Age, this should result in heavy damage if not death, as long as you have all the proper upgrades (bloodlines, scout armor, fletching). Without every one of those, your feudal play is out of optimal range again. The first attack makes all the difference in my experience.
Tl;dr; Correctly evaluate the map -- openness, main gold placement, and hills surrounding opponent base. These elements amplify full feudal, openness with main gold infront his base being the things you want to take advantage of. If there are all 3 of those, there is a strong chance for a brutal knockout if you play it correctly. And start your first attack around minute 16.
Edit: Forgot to mention this. If your civ doesn't have bloodlines, I wouldn't advise going for full feudal. Its too weak then, and even with scout armor your scouts can get shredded from archers/spears (hills being a huge key if they are on map). Franks are the only exception here, as I believe their scouts should have enough HP with armor to survive downhill vs ranged army if such an engagement occurs. Make sure your civ has bloodlines, otherwise you should have another plan.