There is a disclaimer in each of the Ultramodern5 and Apex rulesets' preambles stating that they do not aim for balance. Has anyone played them extensively enough to know if there are any major balance issues? Overpowered characters sometimes destabilize our group's whole campaign.
I've played Ultramodern. They have a strong emphasis on classes being the main source of power, which reduces some balance issues - items are rarely OP or balance-shifting.
They have some "roll on random tables" things, so there's an inherent balance issue there. Your character may be a one-eyed, disabled person on the run from the mafia, but they'll also have a bunch of extra feats that make them very powerful in a certain area. With each windfall comes a tragedy, but it's easy for those to be unbalanced.
The level-up customization archetypes are super niche, so it's generally very easy to challenge your PCs with any problem outside said niche.
The difficulty in building your character well and the random elements are the key things to carefully control to minimize balance issues. You need to be very careful with those two things, and then it's good and you can have fun.