I have been mountaineering for close to 2 decades and boating for the past 5.
PLBs, and other satellite communicators like Garmin InReach (the one I own) also need a line of site to the sky. Coming from mountaineering where you are often under tree cover, I can say they kind of suck, you really need to get above tree line for these things to work with any sort of reliability. Any iPhone 14 or newer also has satellite SOS capability, they also need line of sight to the sky, and in fact the on screen instructions that help you guide the direction of the phone actually make the ability to pickup a signal even better. Given we are talking about marine use, line of sight isn't a concern for either device.
The difference between these 2 things is that an iPhone has a 24 hour battery life, and requires a lot of button presses in order to send a satellite SOS message (ever try to use a phone in the rain?). PLBs or InReach are built to be more rugged, have a longer battery life, and have 1 button SOS functions built right in. I have many friends in SAR, and they will say don't trust an iPhone because they also don't want some random urban walker to scale a mountain and have a false sense of safety. It's not the performance.
That being said, I've decided not renew my InReach subscription this year. The amount of mountaineering I do is significantly down, so most of my time is spent boating. I have a floating, waterproof, VHF radio with AIS and DCS distress (single button much like the PLBs and InReach). Most of my time is spent in the San Juans and Gulf Islands like you Mastercraft, where I feel pretty comfortable about VHF performance with relay stations that the US and Canadian CG have in place, and the number of other boaters that are typically in the area. My iPhone will be my fallback, and depending how much of an emergency it is, I have Starlink if I need to get ahold of someone as well.
If I intend to go offshore, I'll add a PLB or re-enable my InReach, but for now I think iPhone + VHF is a good combination.