Do yourself a favor and do not book long term in the Valley in the (I'm guessing likely) case that you do not enjoy it. It is a long narrow city that runs roughly from Mission (west) to San Benito (east) down to Brownsville (south). In much of this area you'll see graffiti, broken windshields, bars on windows, fences around stuff that have value, and litter. IMO you'll see more of it as you head west and south in the area. Seems like Weslaco is the dividing line.
Take a look at some of the government statistics and I think you'll find that the area has a low mean income, high unemployment, and high school or less education. Not the ingredients for an upscale tourist destination spot. I know of no local Texans that go to the valley for a vacation. There is a ton of Winter Texans escaping the cold that are on a budget though.
Is it safe, well that is subjective. Are you safe inside an all inclusive resort in Jamaica, of course you are. Are you safe roaming the back streets after dark in Jamaica looking for that local jerk chicken, well maybe not so much so. Valley is the same way, not many leave the parks once the sun goes down. Might explain why the parks have so many activities to participate in.
Long story short, some love the area, others can't leave fast enough. You'll need to see it to determine which one you are. Just don't book long term if you are the type that can't leave fast enough.
