Why do I go down in three hits but the opponent can take easily 20 to 30 blows before they go down. Awful quests and it doesn't help lock-on is broken with the Blood and Wine update.

Perhaps because you have a save option and your oponnent does not :P
For difficulty i guess

If it helps any, I find the best way to do fistfights is to keep your distance, which goads the AI into trying a heavy punch, which is very easy to counter, as opposed to their jabs. Yes, it does feel stupid that epic monster slayer Geralt goes down after a handful of punches while everyone else has adamantium for skin.

Just be glad the fistfights have real gameplay again, rather than the QTE trash from Witcher 2.

I haven't tried fist fights in B&W yet, but they can be won easily by just counter attacking. Countering at the right time stuns the opponent and you should be free to get two heavy attacks in there. By doing so, you can even win against opponents who outlevel you far.

Big bump, sorry. I'm having some problems with this, though admittedly outlevelled 24 to 19 in Novigrad. I get knocked out after about 3 hits, and can't really do much against him. If I am careful with a hit here and there, he quickly regains stamina and I can't seem to injure him. If I try to get in series instead I can injure him properly, but soon enough he will counter and 1-2 hits later and i'm half "dead".

Is there no way to manage this without leveling up? Looks like levels have a *lot* to say in this game. Could barely tickle some high-level monsters earlier.

No, the level of the player relative to the level of his opponent isn't that important in fist fights. The key is counterattack in any of these fights - the same counterattack which is very useful in swordplay.

An example: Geralt lvl 18 vs. Valgard, lvl 27, DL Death March:

https://www.youtube.com/watch?v=D2iDGlTlEaQ

Don't know if you used counterattack before. If not: Here's the instruction for use:

https://www.youtube.com/watch?v=T0qB6Mwbb8w

Thank you for this, the second video is very useful - or at least looked very useful when I saw it. I've tried it out against bandits and suchlike, but mostly end up getting my arsed whipped when trying to (block and) counter attack. Looks like the timing has to be perfect and there is a very narrow window, so it's hard to get it right, especially consistently.

I'm mostly doing fine in combat otherwise by using dodging and the odd roll (on sword and broken bones difficulty), but counter attacking is something I've mostly given up on. And when I do succeed against bandits, I'm not actually doing any "automatic" damage, like it appears like happens in the video you made. I'm just kicking them away a little bit.

I have tried out "normal" blocking, though, like against a horde of wolves or nekkers. It works really well, but ofc I need to get in damage too, and not just block incoming hits. That can be tricky when surrounded, but later on I'll pick up Aard sweep, which should help a little bit.

Fist fighting, though, looks like a really hard prospect given we can barely take any hits and can't use signs, eat food or take potions.

Not sure if you understood that right. A perfect block does not automatically create a counter-attack that does damage, just the kick you described. However, after the kick, the enemy is unable to do anything for a short time, leaving them wide open to any regular attack, likely even more than one before they behave normal again. So to make blocking work, you do need to attack at least once directly after it.

There is no immediate automatic counter-attack like you could to in Witcher 2. This makes this more difficult to use when you are attacked by more than one enemy, since while the target might be unresponsive for some time, their friends are not and can still attack you and prevent you from making use of the "free" counter-attack effectively. So for dealing with groups of enemies, I would stick to evading in most cases.

Blocking is pretty crucial for difficult one-on-one siturations like the fistfights. If you want to practice the timing, I find stating a new game and doing the tutorial fight with Vesemir helps, since you can do that as long as you like and he does not damage you much if you mess up.