How much better is a 5*+10?

every +1 merge a unit get = + 1 on two stats.

so a 5*+10 gets +4 to every stat.
there is a pattern to which stat gets the additional stat , on the first merge
the highest and second highest stat(at base with no passive skills and weapon).

If I'm not mistaken, a +10 5* has +4 to each stat and this is the case for every unit.

I believe you're correct with the random gains. I think they're random between units, but not those that are the same (i.e. all Myrrh's will have the same ordering of gains). But yeah, in the end they all gain the same +4 to each stat.

Edit: It's not really random. The person above me actually knows how it works. So ignore the random part.

A +10 5 * has +4 to all their stats and there is pattern to the stats a merge gains.
+1= hp and best stat
+2=2nd and 3rd best stat
+3=4th best stat and hp
+4=best and 2nd best stat
+5=3rd and 4th stat
and so forth.

stats follow a kind of complicated trend. At +10 a unit will have 20 more bst than at 0 merge.

Each merge will give pre-decided points. The first merge will increase HP and the highest stat of the unit (some may argue that it increases the highest two stats, but in my experience it will select hp even if two other stats are higher.)

So, for example, a Celica with +Atk and -Hp will get +1 Atk and +1Hp for the first merge. The second merge will add +1 to the next two highest stats, in her case being one being speed. Celica, however, has the same stat in def and res, so the game will decide one. I BELIEVE because she has a tome the game decides res for her next stat increase, giving her +1 to res. The next merge, will then increase Hp and Def by one each, since Def hasn't been increased and Hp will always be prioritized if it has not been increased more than other stats. At +4 merges you will gain another atk and speed point. And so on and so forth.

With this you can kind of decide how many merges will be most effective. At +2 you will get an extra point in the two most important stats of your unit. Sometimes going to +3 will not be useful, since it adds one point to the lowest stat and one to health. The same with the +5 merge and +10 merge: these merges will give points to the lowest (and arguably the least important) stats of the unit each time.

All in all, most units at 4* compared to their 5 gain a 2-4 point different depending on bane boon.

Merging overall will also buff each stat individually by 4 over time.
Merging always does
The two best stats on +1
The two middling stat +2
And the worst stat and best stat on +3
2nd and 3rd best on +4
4th and 5th on +5

And you could imagin how the process goes beyond that. Overall, you gain 4 extra points overall through merging on either rarity. And the extra stats for the highest rarity and extra merging will total to about I dunno...

Most boons have 3 points extra, maybe the rare superboon might come into play (+4 points), that's about 7 to 8 points of extra stats if you are willing to invest.

Currently my Mae +9 is a broke man's Linde sitting at 42 speed because
Summoner Support, Refinement in Speed for Tome, and Stat Boon.

And her speed sits at 31 neutral.
So I'll let you do the math and ask "does this guy love Mae?"