As I’ve written, potential evidence must be relevant and there must be a foundation laid for it before it is admissible.
I don’t know of any evidence in any type of case that is “completely admissible” without being relevant and being properly authenticated.

If one begins from the premise of UR Wrong, that text messages are “completely admissible,” the next question one must ask is how they would be introduced into evidence.
This seems to be an issue Mr. Wrong overlooked. This could be done in a number of ways.
One could bring the phone and enter it into evidence. I think this is a poor choice for several reasons including the fact that your phone would have to stay at the court for the duration of the trial and any subsequent appeal.
One could subpoena the text messages directly from the service provider. I’ve done this in the past and can tell you with certainty that the actual text messages on the server are only kept for 2-3 days then deleted. I was told (by verizon, I believe) that this is due to the sheer volume of text messages sent on a daily basis and the finite server size would render it impossible for them to retain all text messages sent for much longer than that.
Another option is to print the text messages out somehow. Perhaps by emailing them to yourself (not all phones have this capability), or by having a data recovery expert pull them off the phone and print them and then come and testify as to how it was done and that they remain unadulterated (pricey). Some people plop the phone on a copy machine (usually turns out looking not so good if readable at all).
No matter what method one uses to actually turn the text messages into an admissible format, each method can be attacked in voir dire. Sometimes successfully, sometimes not.

And yet another hurdle remains: How does one prove who was typing on the other phone at the time? If the other party gets up on the stand and states his or her phone was lost/stolen the day before the horrible text message was sent, produces a police report to that effect (it’s pretty easy to file a police report for a lost or stolen cell phone) and testifies that he or she did not send that text, you’re going to have problems with both admissibility and weight.

Nevertheless, I invite the reader to check some other sources, and I invite “UR Wrong” to qualify his (or her) statements and provide some context for those comments: