HP/MP on hit can be reliable in PvE and sometimes better than using regen only but other times, a decent regen will work well, goes great with desperation mythic for discordance or other type builds that use mana. HP/Mp on hit can save slots though and be effective. Glasscannon, barbarian, push the limit all are workable with them too. Energy, Fury and other resource mythic affected by hp/mp on hit but energy and fury isnt affected by mp and hp regen since it has its own recovery of 30% per second. Using up a fair amount of energy while not running out can gain a lot of damage and keeping fury full by spamming primary skills while using that 50% dmg for special skill is good too.
Bloodmagic can use both hp on hit and regen and it converts the mp on hit and mp regen to HP anyways. Alchemy likes regen and mp/hp on hits, harmony mythic too. Equivalence benefits because it keeps the bar full for best dmg reduction and reduced resource cost.
In PvP , the regen such as from permafrost bonus seems more reliable than using MP/HP on hit generally because of how small MP/HP you gain per hit from mp on hit but regen can give back a fair amount of HP/MP. I did start off with MP on hit in PvP though which did recover a bit of MP when my orb hit so fast but regen made HP/MP recovery quite quicker and easier.
Glasscannon is normally super worth having in PvE because as you go higher floors, HP starts to matter less that dodging and making sure you don't die matters and a good defense tends to be a great offense. 50% usually i would use but 100% works great too, even though you would have 1 HP but sanctuary Mythic is good mythic to survive deaths once every 30 sec duration. Barbarian is a good affix in increasing dmg too but you may find less MP to work with. Push the limit increases MH and OH% dmg of many things at the cost of increased mp costs.
For pvp, you could still use glasscannon but HP does matter a bit more as well as MP for barbarian but push the limit is one of the easier ways to really gain a lot of dmg as the mp cost increase isnt too bad but you can gain a lot of dmg.
Basically it depends on what type of build you make.