The only issue on the SE, that I'm aware with on T-Mobile, is that it is lacking Band 71. Band 71 is the 600 Mhz spectrum that T-Mobile is just starting to roll out nationwide, and that they hope to complete over the next two years. Also, if the merger with Sprint goes through, the SE doesn't have most of the Sprint LTE bands (particularly band 41) -- ones that T-Mobile will likely use to further improve coverage and speeds.
Of course, most of T-Mobile's current iPhone's have this same issue -- no iPhone has Band 71 currently (only a handful of phones do support band 71), and most T-Mobile iPhones don't have the Sprint bands (you need the Sprint or Verizon iPhone). On the flip side, if the SE works well for you now, it should continue to work well in the future; you'd likely see some speed and coverage improvements with a new phone, but your current phone shouldn't get worse.