I agree with that. I can't see why some character backgrounds should be rewarded with extra goodies, while others get nothing. If there were some parity with backgrounds, that would be one thing. Having different Houses get different bonuses is generally okay, for example, because there's some parity in that. Everyone belongs to a House and presumably gets some benefit from their House. But for someone to be able to say that their character concept warrants them to receive significant bonuses whole other concepts don't seems unfair.
But then I've gone and used what one of my law school professors called the four-letter F-word. "Fair."
Why should one character be compelled to perform a season service for their House year? (Tremere) The setting answer is that the House provides them support in exchange for that.
A lot of the mastery abilities aren't all that great, either. Stalwart is pretty great, to be sure. But the idea is fairly consistent, if poorly executed, getting access to additional "power" requires some sacrifice, see above with Tremere, and then there's the whole Cult of Mercury Societas. The idea is that if you want some extra power, you have to fulfill some obligations, somehow, and they're described as being fairly light.
House Tremere, unlike your covenant, has copies of the best available Roots for every Art (they traded the use of their junior magi for them), along with several Branches that they managed to get off of Durenmar. In fact, they probably copy said Branches internally at Coeris - while they don't sell copies of said Branches, they do make them available to more Tremere magi than they would be at Durenmar (which has an unofficial policy of not actually making their good books available). They pay handsomely for a library of quality tractati. No Tremere magus should have to study from vis.
House Tremere has longevity specialists on contract. (No, this isn't specifically on the list of available gifts, but come on.) When a Tremere magus turns 35, they can be assured that their exarch has already marked them on one such specialist's calendar and that their LR is paid for.
Coeris has a bigger stockpile of magic items than anywhere except Verdi, tailored to a variety of uses. Need one? Just sign for it. (Youngsters, expect to wait a bit.)
In short, House Tremere is a lot closer to that ideal Order that works together and shares resources for the good of all than...well, the regular Order is. I would assume that Tremere magi can count on decent compensation for the season they spend in service to the House.
To be honest, it's OK to let some character backgrounds get goodies as long as those goodies cost something. In the case of skill masteries, those masteries require xp which have to come from somewhere. To get lots of skill masteries would require you to take Flawless Magic, which uses up your slot for a major hermetic virtue - so preventing you taking Mercurian Magic or the Tamed Magic at character creation. Letting Tremere get extra stuff in exchange for service is fine, as long as the two are roughly proportional. So your other character can't start with some particular bonuses - there are a huge number of ways of achieving mastery in your chosen sphere of magic, so you could instead focus on more Arts, or tweaking your lab, or joining a Mystery Cult, or adventuring for a lost magic secret, or.....the list is infinite.
Your magus will never be able to do everything. If the range of backgrounds means someone might be slightly better at one aspect than yours, decide whether you want to specialise in something different or ask your SG for an adventure that will let you gain access to something that will compensate. Darius of Flambeau does not care who has access to strange skill masteries or mystery cult virtues - he cares about whether he gets to be a Hoplite, and getting enough Creo, Ignem and Penetration to hunt the evil-doers down.
But it is a fairly common interpretation. I run ArM5 Tremere much the same way, and most everyone I have encountered seems to have similar expectations. I do admit that I handwave the service season. More accurately, I sort of forget about it. Then again, I let players start off at fifth level and said Tremere enters play as a Councilarius already. I sort of play it as "you get what you give". The more you play your character as being helpful to your House or Societas, be it Tremere or Mercere or the Knights of Seneca in Flambeau (my saga does vary).
Yeah. Well, I respond with "what Ken and Trogdor said".
Goodies and perks and special aspects of background are all well and good. And having goals and aspirations is fantastic. But these are just simple stupid spell mastery abilities we are talking about, not some special mystery or breakthrough or even anything all that powerful. Low hanging fruit. It is like an employer telling skilled staff they should be happy at minimum wage.