The problem comes with mystic adepts... because they have access to ALL the metamagics both mage only and adept only. And there's always relevant and useful metamagics for them to take. You effectively allow them to raise their adept powers on the cheap while also raising their full spellcasting magic, and they can then buy the metamagics they like for a bargain basement flat 15karma cost each.
Here's why I say this is problematic. karma costs. They weren't changed or even given any thought when the SR4a attribute costs were made.
Initiation costs are far lower than raising magic scores generally.
Your points are valid, but on this detail I feel the need to point out that, as the system currently stands, initiation doesn't get you extra magic points or PP. It only raises your maximum magic stat, and then you have to spend additional karma. That becomes very steep very quickly, and I think the issue here is that Adepts (and maybe magicians) can't keep up with a cybered street sam who's found a black clinic.
This provides a nice cheap way for pure adepts to buy PP's while still subjecting those powers to the max rating == magic limitation. Generally this results in adepts buying more varied powers *and* simply dumping all their points into one or two.
You still have that limitation of max rating=magic, so unless you spend that steep cost of bumping up magic also then you don't have "instant superman, just add initiate". Some very respectably powerful characters possibly, but the same goes to Sam with his black clinic, and he doesn't have to wait to have enough karma. With what you're describing of initiating and buying PPs, it seems that (like with active skills), you'd run into a ceiling and have to diversify, which I do not see as a bad thing. I also figure it's inevitable that somebody's going to find a favourite adept power and dump PP into that before they start diversifying. That's where having a gentleman's agreement or using Mirikon's phone book comes in.
you can know a maximum number of metamagics of initiation + magic
I thought the limit was based on your initiations. I looked up on S4A 198, but it doesn't say specifically. I did find it in SM p52, which I think is a little overly generous. Maybe they'll reduce it to Initiate+half magic or something in 5E. As it is it
does allow very low-initiates to gain an extremely wide power, though you've also got to find a teacher for those metamagics, which the GM should not be easy with.
Magicians want the initiate grades badly so buying metamagics without the free initiate grade at the same time is a bad deal. they almost never make use of the buy for 15karma metamagic. But for mystic adepts as soon as you provide the alternate method to obtain cheap PP. suddenly this becomes a big deal. Because for the cost of raising their basic magic score, they can buy the reduced cost PP along with the increase in initiate grade, still get the metamagic. and pay less than if they had paid to raise their raw magic score from 5->6 in the deal. And still have room to pay to raise their magic score to boot
I have agreed with your points up until here, and this is where I think we disagree.
First, I will acknowledge that it is cheaper to raise initiation, with cost=10+(grade*3). For the third initiation, that's 19 karma (keeping in mind that you've already spent 29 karma just on initiation, while buying magic from 5>6 is 30 karma). Three potential metamagics without having dipped into any funny rules. However, this setup is the baseline for magicians and works quite well for them. Adepts have very different function, and so there's no need to lock them onto the same railroad tracks. It seems that neither of us think that this or most other "anti-nerf" optional rules would hurt them or unduly unbalance them, so let's set them on a shelf. Mystic Adepts are indeed a stickier problem because they have the potential capability of magicians - with all of the required karma costs that involved, which would not be insignificant - and also are trying to keep up at least partially with adepts, who are spending their points on other things. The way I see it (and possibly others like JustADude before), they're trying to take the same handful of karma and throw it more directions.
Effectively they're only paying 15karma more than a full mage per point of magic and getting the full adept powers to boot
It's not like they're geting them for free. 10+(grade*3) just to initiate [same as the others] with 15 karma for metamagics [many of which are dependent on good initiate grade for effectiveness, as you already pointed out with magicians], and on top of that there's 15 karma for 1 additional power point. At grade 1, that's 43 karma. There wasn't a limit stated, which might be your concern, but just maximum PP=magic+initiate grade would be enough. That way they're either paying for initiation or for magic, both of which are expensive (buying up to grade 10 is 265 karma, without any other costs tacked on). As long as you keep the separation for Magic used for Adept powers are not available for spellcasting, I don't see mystic adepts being all that easily overpowered, especially because Adepts are only using their PPs for becoming urban ninjas and Magicians are only using their karma to initiate and boost magic. If you want to drop the "15 karma for a new metamagic (not including one potentially gained during initiation)" would that smooth things out for Mystic Adepts? Keep a limit that on initiating you can only go for
either a metamagic or an extra PP and you won't have übermystics playing ultimate magicians and ultimate adepts. They'll have to pick being very good at one, or moderate at both.