The PowerBook 500 series w/100MHz PPC upgrade isn't too far off the mark of performance for a 5300, and that's partially because the 5300's architecture basically adapted the PPC bus to old '030-based chips to shorten development time and keep costs down, but also to share chips with the Duo 2300. The 500 series did the same (it adapted the '040 bus to an '030 bus in part to keep costs down by reusing old chips, but also so they could use the same chips in the Duo 280 which was required to use the '030 bus because that's what the Docks used). Basically, if you're looking for a reason that the '040 and early PPC PowerBooks were kind of underwhelming, blame the Duo line.
If I had a choice, I'd prefer a 500 series w/PPC upgrade over a 5300 for a variety of reasons but mostly because the 500 had more built-in features and had dual-battery support. The only real advantage the 5300 has is more RAM (up to about 64MB vs. the 500's 36MB) but unless you were really wealthy at the time nobody was rocking more than 32MB anyway.
The 5300 had its slightly squished dimensions because Apple execs at the time wanted the 5300 to be, paraphrased, "the smallest 'full-featured' laptop on the market." Of course, 'full-featured' apparently meant no CDROM support (they were supposed to have optional 3.5" CDROM drives but those were never released publicly, and besides, who used 3.5" CDs?), no modem, no Ethernet, and only a single battery, so I'd say they missed the mark.
The 1400 was internally identical to the 5300, it just had a different case and a swappable CPU. The 1400 was originally designed with dual-battery support but it seems Apple didn't want competition with the 3400, which was the more expensive "pro" PowerBook but did not have dual-battery support, so they dropped it. I'm not sure if it could be added back in (the pads are still on the logic board) but maybe a fun project for someone if it's possible.
Anyway the most common problems for a 5300 are the power jack, hinges, and display caps (especially on the 5300ce). These aren't 5300-exclusive failures (the WS/PDQ PowerBooks also had problems with power jacks coming away from the board, and hinge failure, and the 13.3" displays were notorious for problems) but they're what to expect, assuming you don't also run into generic old-tech failures like battery leaks, LCD vinegar syndrome, fragile plastic, etc.