I think the only main disadvantage to the low voltage LED conversion is the time and cost of running a lamp out to the pool and installing a transformer. It has to be a POOL-code rated transformer and you’ll probably want at least 300W of available power if you plan to do any landscape lighting as well. So that transformer can easily run you into the $500 range. If you’re handy and can do your own electrical work, then you can save on hiring an electrician.
Also, by way of a anecdote, I feel like we have way more posts about failed low voltage LED products rather than the 120V replacement LED bulbs. LED lighting products like the Pentair IntelliBrite line can easily cost upwards of $1000 for pool and spa lights and their lifetimes seem to be a few years at best before one of the color segments burns out. Globrites and the niche-less products all require special housings that are not compatible with standard wet niches. So your replacement costs are going to go way up.
Any savings in electricity costs might be wiped out with installation and replacement costs associated with a low voltage conversion.