Hello and welcome!
To answer your questions, considering my experience:
1) can it easily be converted to modern day?
That depends, I'd say yes. Thompson could easily be used as M4, it has shotguns, double barreled shotguns, even chainsaws, two types of pistols, rifles, dynamite (read grenade and you're good to go). You could easily play moderns without any addition to rules except for renaming them, but could also, say, add range to thompson gun and make it more like an assault rifle compared to submachine gun. Depends on variety you'd want.
2) are the bad guys just cultists most of the time?
Again, it depends. I don't have any cultists painted, so I rarely use them. You can pick among over 20 profiles, at least half of them are low points, so could be used instead of cultists. There are some scenarios where lurker (evil guy) has to have more models than good guy, and where all the lurkers must be humans, but not exactly cultists. There are rules for henchmen, who are humans, for example. Cultist could be called militia for your modern setting, its just a lowly motivated poor chap.
3)how is balance I really don't like when I can just romp my opponent OR vice versa Laugh.
That is just me, but I would say it depends, again. Game most certainly needs to be played a bit more in a story feel. I'd say any game can be broken, here you can pump your constitution sky high, so only really high level monsters can kill you, and same can be said for monsters, which would mean only high power weapons would be used. I really like the idea of dual pistols so when I play, we tend to use lower damage weapons to keep it diverse. If you want to get it broken, you can break it - but again, what I really, really love about this game is the feeling it gives, really fun.
4) this applies to part 3. are my agents with m4s or aks going to just wipe the floor with melee monsters?
No. Sure, you can mow down puny cultists, but tougher lurkers should find a way to get in close combat. Do keep in mind it is easy to get removed from table (but not actually die), so cover is really needed in this game. Playing board is only 3x2, so its easily filled. Obviously, if all your agents fire on 1 approaching monster, it has no chance, but, again, I've found avoiding melee quite hard.
5) can you create your own monsters?
And yet again, it depends
. Profiles are very generic, say "flying nightmare", so you can use ANYTHING you want for it. I went as far as using wolves instead of raptors, woolly rhinos instead of triceratops etc. Each monster can be given 5 skills, which makes it a bit more specialized so you can kinda create your own monster, but there are no rules by which you could make a monster from a scratch. Nothing stops you from making something unique, giving it rough points cost and posting it here, so we can all play test it a bit.
I mean, I find it hard to answer your question - I came to this from Flames of War (not even 40k!) and I must say...It is a beautiful, beautiful world. This game is very simple in its core and you can make anything out of it, really, anything. I am most certain that I will do sci-fi bug hunting and I am working on zombie apocalypse, while some other guys are thinking about playing it in dark ages. As for the monsters, I've just wrote in topic below, its more than 20 different profiles available, so in one way or another most of what you could think of is covered. Except vampires and mi-gos