Certain types of projects are nightmarish, but it works fine for others. Its greatest weakness is that its handling of global events is poor, and certain things become rather messy. You can for some reason give objects families to have a single event code for multiple objects (e.g. collision detection coding for enemies is probably going to be identical for enemies that behave similarly, except with a different enemy, so you can create a general "enemy" family that will handle all of them) but you can't use them globally (meaning you have to either copy and paste the global events and swap out the object, or have to copy/paste the events into every individual level).
Its built in platforming movement shouldn't be used, because it's awful (I can't remember what specifically was bad about it, since it's been a while, but I remember that it was bad) though there's an extension object that's okay to use.
If you're trying to make a Mario clone, it's probably fine. If you're trying to make a Metroidvania, it's gonna start getting a little screwy but it should still work. Some puzzle games are probably fine. I'm not sure how well a lemmings replication would work - I'm not sure if it would cooperate well with terrain removal.
Don't even try to make an RPG.
Its strong suit is that it's fairly simple for simple projects, so it can be decent if you're just starting out, as at least some of basic techniques will transfer to other, less user friendly software. Just note that if your project gets complicated, things get convoluted because it doesn't really offer many ways to keep things organized.
I've never tried Game Maker before so I can't comment on that.
TL;DR
Clickteam Fusion is usable at the entry level, but there is better software to use if you're making a serious project.
For general usage purposes, you may wish to consider Construct 2. It's not as user friendly, but for more complex projects its better handling of events will make things much easier. (You can have multiple levels share the same event sheets without committing literally every screen in the game to having those same events - which means your title screen doesn't need to share the same code with the rest of the game anymore! - this would also allow things like having event sheets coding for enemy AIs then not loading them when they're not needed - not to mention you can keep things a bit better organized that way)
For RPGs, I'd recommend any of the RPG Makers except VX. The most recent version is currently RPG Maker MV. 2000 and later are all available on Steam, with 2000 and 2003 being relatively cheap (pretty sure XP is pretty cheap too). If you're using game making software instead of programming everything yourself, there's really not much point in making an RPG engine as opposed to using this. There are a few limitations but most of them aren't a huge deal on the latest versions, since functionality can be changed with scripts (XP, VX, VX Ace) or plugins (MV; same function, but made easier to install and configure). Screen resolution used to be an issue in earlier versions but has been resolved as of MV. The nice thing about them is that if there's an advanced functionality you need that's not built in, you can write your own code for it (in many cases, someone else has wanted to do what you're trying to do and released a plugin you can use). It's limited in terms of built in functionality but it's pretty simple to extend it because of the plugin system.
Note that projects made with RPG Maker carry some stigma, because the ease of use means there are a lot of low effort works made with it out there. It can be overcome fairly easily though simply by putting in effort. It's usually as simple as "Don't use the default graphics for serious projects." Also, put in effort, like you should when you make anything you publicly release.
