FEW - I am not sure what you mean. The combo system is the same. After any number of normal attacks (Y button) you can press the X button for a strong attack that will differ depending on how many times you pressed Y before. Unlike in Hyrule Warriors, there are skills outside of weapons that each character can equip and all characters will have access to all skills eventually; there's one skill that increases the attack speed by 50%, so executing combos will be quite fast.
The critical hit bar works differently. In Hyrule Warriors you had to dodge an enemy attack to see it appear above the enemy's head. This is still in FEW, but in most cases a YYYX combo will make it appear too, so you can go on the offense. If you have an advantage in the weapon triangle, then four normal attacks will make it appear, so a YYYYX combo shows the critical hit bar and immediately depletes it.
SMO - Mario still takes forever to stand up, although knockdowns aren't as frequent as in previous games. I don't remember any spikes right now; there are bottomless pits, lava and poisonous lakes which all result in instant kills and you'd expect them to do that regardless of invincibility.
FEW - So it's the same as in Warriors games.
SMO - It's an improvement then. Instant death from lava is so much better than watching Mario bouncing hopelessly until he runs out of health.