I rewatched all of that shit awhile ago. Xena and Hercules are on par with the original Star Trek for me - yes, admittedly campy, and low budget SFX, but the writing is decent enough, and it's just good fun.
Highlander on the other hand didn't hold up at all - and this is coming from a guy who bought all 7 seasons on VHS. That shit is nigh unwatchable with how bad it is. Not really sure what's wrong with it either - I mean, evil of the week certainly gets old, it's pretty formulaic, but it almost like there's nothing in the series to give a shit about at all.
As someone else said already, the main problem with the show is that it constantly mentions immortals getting "power increases" from their foes, but never explains how that actually works. At the end of season 3, Duncan defeats this really powerful menace after an entire story arc, and at that point he's defeated a few very tough and legendary immortals. So then in season 4 he basically crushes all of his opponents because he's so powerful by that point...except it doesn't really explain how it works, yet again. Then in season 5 he has a few really challenging fights again, and wins those, so he should be getting even more huge leaps of power from that right? Well who knows. A guy who has the power of Kalas, Consone, Haresh, Martin Hyde, Xavier St Cloud, Grayson, and KRONOS is, in theory, gonna be basically invincible at that point right?
The movie Highlander Endgame attempts to actually explain how the power increases work, finally, but not really. They try to measure the power levels of the characters with their total kill numbers, but it doesn't make a whole lot of sense still. Duncan has like 180 kills and is pretty much evenly-matched with Connor and his 250 kills. That could be Duncan making up the difference by being more skilled at technique, or it could be that Duncan's 180 kills had some higher-quality opponents in it.
But then later in the movie we go from kill count mattering most to quality of kills mattering most, our heroes combine their 250 and 180 to get a 430 and be able to fight the bad guy and his 660. ...I mean it's all ridiculous and maybe the point is that it can't be explained and they're just taking shots in the dark the same way we are.
If I had to come up with something that made sense, I'd say it's based on kill count but you also get all the kills of those you've killed, and all of THEIR kills. This would explain why immortals pretty much never bother with "young ones" in the show. Like veteran immortals have basically zero interest in new immortals, or immortals that haven't won many fights yet. This is very consistent throughout. It must mean that they really do have a power quotient of like, 1. Duncan with his 180 kills, if his opponents on average had like 15 kills each, then that means he's got a good 2500+ score. Maybe he has a few high-value foes, like Kronos who was around 4000 years old (maybe even older) and likely had several hundred kills. So Duncan is actually in the 6000s when it's all added up. A noob immortal is going around with... 1.
So what does all this power even DO? That's the other thing they never explain. Who knows. I don't think it affects physical strength, the person's physique does that. Immortals work out all the time. So I'm guessing that it confers personality traits and knowledge / ability more than anything else. You might now know a move you didn't know before, or have a deeper knowledge of how the world works. ...which means it really isn't a power level at all, but rather just a consolidation of knowledge and abilities.
