As with all radios, if you actually spend the time to get to know it (many DON'T) including the menus, you'll get the best out of it. But this takes time/operating hours on the modes you are intersted in. The FT-991/A is a great little radio with a few idiosyncracies (like the VFO A/B indicator silliness) and a slightly obtuse menu system. But once I got to know it, it works fine - and is very handy in portable ops.
As mentioned above, the FT-991/A and TS-590S are an apples-to-oranges comparison, as the FT-991/A includes multimode (AM/SSB/FM/digital) VHF and UHF, which, by itself, is worth the price of the radio. While the TS-590S is HF/6M ONLY.
So it's really a matter of size/ergonomics vs. VHF/UHF capability. Because both are pretty good (though not stellar) on HF. For "stellar", go get the FTDX-10 or -101.
That's pretty much the long and short of it.
Brian - K6BRN