Pre-promotes in almost any Fire Emblem game are bad. 

However Radiant Dawn has 3 tiers so pre-promotes are only skipping 1 tier of levels and have plenty of time to catch up.  Plus once you get into the later portions of the game, almost every unit you get is pre-promoted.

Except in SD, don't use prepromotes. Also, I remember playing FE8 till every character had maxed stats. Not just level 20, but max stats in every area. Took so long x.x

I haven't played Radiant Dawn yet, but I have a question. Is the leveling system like Path of radiance where you get extra xp and then use that to level up characters or do you have some kind of catch up battles to level up your weaklings?

Three tiers? So... characters can get up to sixty levels?

Yes, you gain BEXP during the game however, unlike PoR, Radiant Dawn's BEXP gives a fixed number of +'s per level (3). 

This means that it's awful for normal level-ups (1-15) but excellent late tier (15-20) because you're guaranteed to get that 3 and can use this to max out your stats with weaker growth (which would otherwise not likely gain +'s from real levels).

Yes.

I had thought FE7 was very easy compared to the GC and Wii versions. Try going through Radiant Dawn on the hardest difficulty, you will spend months going through the first 10 levels.

Three tiers. Gotcha.

So does stat growth work differently? I know in FE7 there's a certain percentage chance that a character will gain in a stat every time they level up (like Hector has a 40% chance to gain in Strength every time he levels).

Iirc Micaiahs team will indeed promote twice. I think Ike's team is already promoted once (which makes sense seen Path of Radience).

Radiant Dawn has 48 chapters, so there's loads of room to gain levels.

No, the basics are the same. Every character has different growth stats for each stat. Over a whole game that would usually mean you have (stat growth*#levels) character. The RNG can screw you though.