Safest tunes are dyno tunes. Also more money.. Mail order aren't made specifically for your car so more risk there. But I haven't heard about any major problems from them on our cars yet so.. And a lot of people have them. The difference the tunes give you can definitely feel. Is it worth it? If your tracking your car I'd say yes but if it's just street driving then eh.. You may get to 60mph 1/10 of a second quicker so if you think that is worth it then go for it.
every car is different, some cars respond better to tunes than others. it just depends , i dont know about the 3.7s but i think they do ok.
it probably wont be worth the voided warranty though. In order to really feel something you would need 20hp at least at the wheels, and even then its not a huge change. I would want 30-40 at least to even consider voiding the warranty
The ford dealer I bought my car from had a tuner in there showroom. It was on display with all the other factory options and upgrades that you can purchase with most new cars. I asked if the tuner would void my warranty and was told that as long as the car is not tuned when I bring it in for service I will be fine. The chances that the shop mechanic can prove that whatever issue you are having was caused by a tune is very remote.
Now, I do have enough mechanic friends to know there are some things that a good mechanic can look for. the big give away is the internal memory of your car's computer that keeps a record of the last time any change was made. It doesn't keep a record of what was changed but it will show that someone was hooked up and making changes. I used to hear about people getting burned by this but told there dealer that they had an auto shop clear a TPMS sensor code or something stupid. Sometimes it works, sometimes it doesn't.