Mythras appears incredibly detailed and has apparently taken immense work. It's a bit too complex for me though, but I bet there are many who enjoy it.
It does appear daunting, but it's actually quite simple and very logical - though I grant that it's easier to understand if you've grown up with RuneQuest. And if you're just using it for skirmishing, you can completely discard all the role-playing stuff and - of course - all the magic.
A few years back, my son and I played through one of the 'combat modules' (the one with the bear) and found it surprisingly easy and intuitive.
Also, there's an
online encounter generator for the RPG that gives you stats for various combatants at different levels of proficiency, which allows you to create sets of opponents more or less instantly, complete with their skills, gear and arms and armour. It's got loads of RuneQuest fantasy stuff in it, but it also has quite a bit of ancient-world stuff too: search for "Roman", for example, or "gladiator". If you search for "hoplite" or "peltast", it'll bring up lots of Gloranthan types that can be used too (Lunar hoplites, etc.). That takes a
huge amount of work out of the rules.
At heart, it's a roll-under percentage system with criticals being a tenth of your success target (so if you need 60 or under, a critical is a 6). Armour absorbs damage, hit points come off each location, and that's about it. What gives it the tactical element that RuneQuest lacks is the ability to choose the outcomes of your criticals - so that you can try to knock a foe back or down, or disengage or hit an unarmoured location or disarm your foe or whatever.