From this and your other questions, it seems you're confused about pitch, tension, string gauge and tone.
Pitch is a function of:
- the gauge of the string (thinner = higher pitch)
- the tension of the string (more tension = higher pitch)
- the length of the string (shorter = higher pitch)
Let's assume the length of your instrument is fixed and not something you can modify. What's left that you can adjust is the string gauge (by replacing the strings) and the tension (by tuning).
If you replace your strings with thinner ones, and tune to standard tuning, it will be at a lower tension than if you'd used thicker strings. This may be what you want
Same strings, lower pitch
If you don't replace your strings, but reduce the tension anyway, you will be playing at a lower pitch. It will also have an effect on the timbre of the sound, and the feeling of the strings.
The more you reduce the tension, the less like a "normal" guitar the instrument will sound. It will become quieter and the attack of the sound will become weaker. Treble tones will die away more quickly. At a certain looseness, the vibrating strings will start to knock into the fretboard or each other. Of course, if you go loose enough, the string will be completely slack and you'll be unable to pluck it!
But let's assume you've not gone to such extremes, and you're just tuning your guitar a few tones flat compared to the standard:
If you're playing solo, there aren't really many disadvantages. The sound your making is subtly different to what people may expect from a guitar (maybe in a good way), but everything harmonises and sounds OK.
If you attempt to play along to a recording, following a chord sheet someone else has written, and attempt to use standard chord shapes, you'll hit trouble.
Say you're tuned a tone lower than standard. The chord sheet says "D", so you fret xx0232 - it's not going to fit with the recording. You're going to need to play an "E" shape 022100 in order to harmonise with the recording. Even then, if the recording is of a musician strumming "standard" chord shapes, you will be playing a different inversion of the chord to them. It's up to you whether that matters to you.
One disadvantage you have here is that many rock/folk musicians choose a key that's easy to play in. For example, lots of rock/folk songs are played in E, so you spend most of the time playing easy E and A chords with open strings. In your flat tuning, you will need to make barred F# and B shapes to play along. Even if you don't find barre chords difficult, it leaves fewer fingers free to add twiddly bits.
There are two ways to deal with this:
First way: Scrub out all the names on your chord dictionary (whether that's on paper or in your mind!) and replace them with the chords you're actually playing in your preferred tuning. So, if you're tuning a tone flat, cross off "D" from the xx0232 shape on your standard chord dictionary, and write "C" instead; replace "E" with "D", "C" with "Bb" and so on.
This way, you retain the one-step translation a guitarist usually does when reading chord sheets -- I see "G", I play this shape. Just that you'll be playing a different shape to what someone in standard tuning would.
Second way: Retain the standard set of letters-to-shapes in your chord dictionary, but do the transposition in your head as you play.
This way, you don't have to relearn the names of chords, but you have to do the extra work as you read music. 'The sheet says "C", so I must play a "D" shape.'
Of course, rather than doing it in your head on the fly, you can mark the names of the shapes on the sheet before beginning. Also many songs have a limited number of chords, so you can just get the set of shapes you're going to need worked out before you begin.
If you're not playing chords, but rather playing a melody, you have a similar issue. If you're tuned a tone flat, then you must play every note two frets higher, to get the same note. Sometimes, though, that's no longer the rational way to play the melody -- you have different open strings available -- so you'll end up playing different fingerings to what someone on standard tuning would use.
If you're playing with other people, all of the issues relating to playing along to a recording remain. There is a further issue of communication. If you tell your accompanist you're singing a song in the key of D - you'd better not open with a xx0232 shape that's really a C chord. The polite thing to do is to take into account your tuning, and always adjust so that what you tell them is correct in their world of tuning, not yours.
Finally, there's always the option of a capo. If you tune a tone flat, and put a capo on the second fret, you still have loose strings, but you're back to standard pitch.
If all of this seems confusing -- well, that's one disadvantage of not going with the standard.