Odds are the line roller on an older reel will have some 'soft' metal (or plating) that may wear down as the braid 'cuts' into it.
Not good for the line to rub across rough metal ;-)
Plus the gearing may not be able to handle the additional stress of using a stronger line.
Any relatively 'new' reel that has a braid line capacity rating should be able to handle braided line within the posted range.
For instance if the reel says:
It should be able to adequately handle up to 20 pound line (braid).
BTW, just because a reel doesn't say "saltwater" doesn't mean it won't work in saltwater.
I put 300 yards of 30# braid on a
BPS Extreme EX50 spinning reel to king fish with and have been very pleased with the performance so far (a year).
At first I just threw plugs with it, but snobbled some this week and caught a king on it!
Attachment 2065
No doubt I was throwing that cig farther into the 15-20 knot wind than the folks using mono, as it only has the diameter of 8#-10# mono!
Luv the feel of a fish running 200+ yards on braid.
I could feel every tail beat and every headshake to the point I thought I'd been sharked several times. lol