Well, sometimes it depends on what it is that's being taken away.
For example, FF6 Brave New World takes away a number of things that technically makes the game more difficult--the level cap is lowered to 50, the number of stat boosts you can get from espers is capped at 25 (you could technically get up to ~80 per character in the original), and a number of really over-powered gear like the Merit Award were removed. It does a lot of other things that balance the other way too, like its heavy rebalancing of enemy power.
Good hardtype hacks do multiple things that are well thought out and mesh well together--equipment access in some games really needs to be managed better (FF15 is a perfect example of what NOT to do--I got endgame weapons relatively easily in Chapter 3), and easy RPGs often have some deficiency in stat balance, whether with the heroes or the enemies.
Bad hardtype hacks tend to just focus on jacking up enemy stats, nerfing hero stats across the board, or some combination of both. The absolute worst offender on this in my experience is the SoM Hardtype on this site (the one with my very negative review) that forgot that damage levels of incoming attacks determined a sprite's hit animations. Thus, heavily weakened heroes could no longer stun lock a lot of enemies, but much more powerful enemies had a pretty easy time stunlocking you with devastating attacks that could kill everybody in short order.
I mostly play RPGs, where there's really a lot of room to tinker with game systems, but I know a lot of other games there aren't as many ways to tweak the game outside of restricting access to items that make the game easier. I mean, LTTP would be a lot more difficult late-game without the blue/green tunics, but not remotely impossible. Every SMB game is also beatable without certain powerups available (I'm not sure, but I think you could come close to 100% in SMW without the cape).